Analysis of a kernel crash

Here is the kernel crash looks like :-

<1>[   49.034499] Unable to handle kernel NULL pointer dereference at virtual address 00000004
<1>[   49.034512] pgd = ed0a4000
<1>[   49.034519] [00000004] *pgd=00000000
<0>[   49.034533] Internal error: Oops: 5 [#1] PREEMPT SMP ARM
<4>[   49.034540] Modules linked in: cmemk bcmdhd snd_usb_audio snd_usbmidi_lib snd_rawmidi snd_hwdep atmel_mxt_ts input_polldev usb_f_fs usb_f_ncm dabridge u_ether usb_storage xhci_plat_hcd xhci_hcd dwc3 libcomposite udc_core configfs dwc3_omap extcon_harman phy_ti_pipe3 phy_omap_usb2 phy_omap_control traceBuf faceplate tun cfg80211 nxp7120i2c loop pvrsrvkm(O)
<4>[   49.034641] CPU: 0 PID: 104 Comm: surfaceflinger Tainted: G        W  O    4.4.45+ #2
<4>[   49.034649] Hardware name: Generic DRA74X (Flattened Device Tree)
<4>[   49.034657] task: eda95b00 ti: edb40000 task.ti: edb40000
<4>[   49.034671] PC is at drm_atomic_helper_check_modeset+0x10c/0x9bc
<4>[   49.034681] LR is at drm_mode_equal+0xb8/0xd4
<4>[   49.034688] pc : [<c045c2f0>]    lr : [<c0479e6c>]    psr: 600f0013
<4>[   49.034688] sp : edb41cb0  ip : edb41c88  fp : edb41cfc
<4>[   49.034695] r10: 00000000  r9 : d160e520  r8 : c0bb528c
<4>[   49.034702] r7 : c0bb528c  r6 : 00000000  r5 : d1048800  r4 : d53dbb80
<4>[   49.034707] r3 : d160e300  r2 : 00000000  r1 : c0bb528c  r0 : 00000001
<4>[   49.034714] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
<4>[   49.034721] Control: 10c5387d  Table: ad0a406a  DAC: 00000051
<4>[   49.034727]
<4>[   49.034727] PC: 0xc045c270:
<4>[   49.034733] c270  e1520003 0a000005 e5983000 e3130010 1a0001bf e5d53006 e383300a e5c53006
<4>[   49.034765] c290  e5943000 e5932208 e2866001 e1520006 caffffdd e5941018 e3510000 c305728c
<4>[   49.034795] c2b0  c3a06000 c34c70bb da000051 e594301c e7939106 e3590000 0a000049 e5972000
<4>[   49.034824] c2d0  e305128c e5943020 e34c10bb e3120010 e50b1030 e793a106 1a0001dd e59922bc
<4>[   49.034854] c2f0  e59a3004 e5920004 e1500003 0a000012 e3500000 0a000006 eb00557e e5943014
<4>[   49.034883] c310  e7933100 e5d32006 e3822008 e5c32006 e59a3004 e3530000 0a00011f e1a00003
<4>[   49.034912] c330  eb005574 e5943014 e7933100 e5d32006 e3822008 e5c32006 e59a3004 e3530000
<4>[   49.034941] c350  0a000115 e59921d8 e592300c e3530000 0a000127 e1a00009 e1a0100a e12fff33
<4>[   49.034971]
<4>[   49.034971] LR: 0xc0479dec:
<4>[   49.034977] 9dec  189da9f8 e595103c e3510000 0a000010 e594703c e3570000 0a000019 e3a00cca
<4>[   49.035007] 9e0c  e3430b9a ebfd469d e1a01007 e1a08000 e3a00cca e3430b9a ebfd4698 e1580000
<4>[   49.035036] 9e2c  0a000006 e1a00006 e89da9f8 e3a00001 e89da9f8 e594303c e3530000 1a00000a
<4>[   49.035066] 9e4c  e5942068 e5953068 e0223003 e313091f 1a000007 e1a00005 e1a01004 ebfffb4f
<4>[   49.035095] 9e6c  e89da9f8 e1a00007 e89da9f8 e1a00001 e89da9f8 e3a00000 e89da9f8 e1a0c00d
<4>[   49.035123] 9e8c  e92ddff0 e24cb004 e24dd00c e52de004 e8bd4000 e5903000 e1a07000 e50b1030
<4>[   49.035152] 9eac  e593312c e3530001 0a00003c e1a09007 e5b9403c e1540009 e5946000 e1a08004
<4>[   49.035182] 9ecc  0a000034 e597a030 e2875030 e155000a 1a000003 ea000023 e59aa000 e155000a
<4>[   49.035211]
<4>[   49.035211] SP: 0xedb41c30:
<4>[   49.035217] 1c30  00000001 c0244278 c045c2f0 c045c2f0 600f0013 ffffffff edb41c94 c0bb528c
<4>[   49.035249] 1c50  edb41cfc edb41c60 c010d324 c01011f4 00000001 c0bb528c 00000000 d160e300
<4>[   49.035278] 1c70  d53dbb80 d1048800 00000000 c0bb528c c0bb528c d160e520 00000000 edb41cfc
<4>[   49.035307] 1c90  edb41c88 edb41cb0 c0479e6c c045c2f0 600f0013 ffffffff 00000051 c0479dc0
<4>[   49.035336] 1cb0  024000c0 c049e130 edb41cdc edb41cc8 c0160d28 c01eabe8 ee801e80 c0bb528c
<4>[   49.035365] 1cd0  edb41d1c d53dbb80 ee1dec00 d53dbb80 00000001 c0bb528c d53db300 d53db380
<4>[   49.035394] 1cf0  edb41d14 edb41d00 c045cbc0 c045c1f0 c045cba0 00000001 edb41d64 edb41d18
<4>[   49.035423] 1d10  c04832f0 c045cbac edb41d3c edb41d28 c0160d28 c01eabe8 ee801f00 00076f08
<4>[   49.035452]
<4>[   49.035452] IP: 0xedb41c08:
<4>[   49.035457] 1c08  eef53610 edb40000 00000001 ee801d80 edb41c3c edb41c28 c0160d28 c01eabe8
<4>[   49.035488] 1c28  ef16df40 c02441dc 00000001 c0244278 c045c2f0 c045c2f0 600f0013 ffffffff
<4>[   49.035527] 1c48  edb41c94 c0bb528c edb41cfc edb41c60 c010d324 c01011f4 00000001 c0bb528c
<4>[   49.035568] 1c68  00000000 d160e300 d53dbb80 d1048800 00000000 c0bb528c c0bb528c d160e520
<4>[   49.035615] 1c88  00000000 edb41cfc edb41c88 edb41cb0 c0479e6c c045c2f0 600f0013 ffffffff
<4>[   49.035653] 1ca8  00000051 c0479dc0 024000c0 c049e130 edb41cdc edb41cc8 c0160d28 c01eabe8
<4>[   49.035695] 1cc8  ee801e80 c0bb528c edb41d1c d53dbb80 ee1dec00 d53dbb80 00000001 c0bb528c
<4>[   49.035729] 1ce8  d53db300 d53db380 edb41d14 edb41d00 c045cbc0 c045c1f0 c045cba0 00000001
<4>[   49.035758]
<4>[   49.035758] FP: 0xedb41c7c:
<4>[   49.035764] 1c7c  c0bb528c c0bb528c d160e520 00000000 edb41cfc edb41c88 edb41cb0 c0479e6c
<4>[   49.035792] 1c9c  c045c2f0 600f0013 ffffffff 00000051 c0479dc0 024000c0 c049e130 edb41cdc
<4>[   49.035822] 1cbc  edb41cc8 c0160d28 c01eabe8 ee801e80 c0bb528c edb41d1c d53dbb80 ee1dec00
<4>[   49.035853] 1cdc  d53dbb80 00000001 c0bb528c d53db300 d53db380 edb41d14 edb41d00 c045cbc0
<4>[   49.035882] 1cfc  c045c1f0 c045cba0 00000001 edb41d64 edb41d18 c04832f0 c045cbac edb41d3c
<4>[   49.035911] 1d1c  edb41d28 c0160d28 c01eabe8 ee801f00 00076f08 edb41d7c ee1dec00 c0244aa0
<4>[   49.035939] 1d3c  d53dbb80 ee1dec00 d1048800 00000001 ee1ded00 edae8000 d53dbb80 edb41d7c
<4>[   49.035969] 1d5c  edb41d68 c048376c c0483158 b6679000 00000000 edb41e2c edb41d80 c04848b4
<4>[   49.035999]
<4>[   49.035999] R1: 0xc0bb520c:
<4>[   49.036004] 520c  00000001 00000000 c0bb5214 c0bb5214 00000000 00000000 00000000 00000000
<4>[   49.036033] 522c  00000000 00000000 00000000 00000000 00000000 00000000 eea751c0 00000000
<4>[   49.036061] 524c  00040004 ee8b3480 00000000 00000000 00000000 00000000 00380038 ee13a5b0
<4>[   49.036090] 526c  ee13a5b0 00000001 00000000 00000000 00000000 00000000 ee240a90 00000001
<4>[   49.036118] 528c  00000000 ee6b9330 ee18d8c0 00000000 00000000 00000000 00000000 00000000
<4>[   49.036147] 52ac  00000000 00000000 00000000 00000001 00000000 c0bb52c0 c0bb52c0 00000000
<4>[   49.036177] 52cc  00000000 00000000 00000000 00000001 00000000 c0bb52e0 c0bb52e0 00000000
<4>[   49.036205] 52ec  00000000 00000000 00000000 00000001 00000000 c0bb5300 c0bb5300 00000000
<4>[   49.036234]
<4>[   49.036234] R3: 0xd160e280:
<4>[   49.036240] e280  00000000 00000000 00000000 00000000 00000018 00000028 00000000 00000000
<4>[   49.036269] e2a0  ffffffff 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036297] e2c0  00000000 f9375000 00004000 00000002 d160aec0 00000003 00000000 c0434614
<4>[   49.036326] e2e0  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036355] e300  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036383] e320  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036412] e340  d160ef40 d160e344 d160e344 00000000 00000000 d16cf180 00000014 00000000
<4>[   49.036441] e360  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036470]
<4>[   49.036470] R4: 0xd53dbb00:
<4>[   49.036475] bb00  00000001 edaaf700 5f434344 4944454d 41465f41 45545f56 552d5453 5f314253
<4>[   49.036504] bb20  49545241 5f535453 2e564146 00666366 00000000 00000000 00000000 00000000
<4>[   49.036534] bb40  00000001 00000000 5f703267 75646f6d 765f656c 445f706d 4d5f4343 41494445
<4>[   49.036563] bb60  3037395f 6c615f31 2e6d7562 0063786c 00000000 00000000 00000000 00000000
<4>[   49.036593] bb80  ee1dec00 00000000 cd7d7000 d1336100 d53db380 d53db300 00000001 d160e500
<4>[   49.036625] bba0  d160e300 edb41de4 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036659] bbc0  00000001 00000000 5f434344 4944454d 41465f41 45545f56 552d5453 5f314253
<4>[   49.036693] bbe0  55424c41 465f534d 662e5641 00006663 00000000 00000000 00000000 00000000
<4>[   49.036725]
<4>[   49.036725] R5: 0xd1048780:
<4>[   49.036730] 8780  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036759] 87a0  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036787] 87c0  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036816] 87e0  00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.036848] 8800  ee1dfc00 00000101 00000003 00000000 00000000 00000000 00000000 dededede
<4>[   49.036877] 8820  00000000 30383231 38363778 00000000 00000000 00000000 00000000 00000000
<4>[   49.036907] 8840  00000000 00000000 00000048 0000f488 00000500 00000510 00000514 00000521
<4>[   49.036936] 8860  00000000 00000300 00000303 00000305 00000326 00000000 1900000a 00000000
<4>[   49.036965]
<4>[   49.036965] R7: 0xc0bb520c:
<4>[   49.036970] 520c  00000001 00000000 c0bb5214 c0bb5214 00000000 00000000 00000000 00000000
<4>[   49.036999] 522c  00000000 00000000 00000000 00000000 00000000 00000000 eea751c0 00000000
<4>[   49.037028] 524c  00040004 ee8b3480 00000000 00000000 00000000 00000000 00380038 ee13a5b0
<4>[   49.037056] 526c  ee13a5b0 00000001 00000000 00000000 00000000 00000000 ee240a90 00000001
<4>[   49.037085] 528c  00000000 ee6b9330 ee18d8c0 00000000 00000000 00000000 00000000 00000000
<4>[   49.037113] 52ac  00000000 00000000 00000000 00000001 00000000 c0bb52c0 c0bb52c0 00000000
<4>[   49.037141] 52cc  00000000 00000000 00000000 00000001 00000000 c0bb52e0 c0bb52e0 00000000
<4>[   49.037170] 52ec  00000000 00000000 00000000 00000001 00000000 c0bb5300 c0bb5300 00000000
<4>[   49.037199]
<4>[   49.037199] R8: 0xc0bb520c:
<4>[   49.037204] 520c  00000001 00000000 c0bb5214 c0bb5214 00000000 00000000 00000000 00000000
<4>[   49.037238] 522c  00000000 00000000 00000000 00000000 00000000 00000000 eea751c0 00000000
<4>[   49.037266] 524c  00040004 ee8b3480 00000000 00000000 00000000 00000000 00380038 ee13a5b0
<4>[   49.037295] 526c  ee13a5b0 00000001 00000000 00000000 00000000 00000000 ee240a90 00000001
<4>[   49.037323] 528c  00000000 ee6b9330 ee18d8c0 00000000 00000000 00000000 00000000 00000000
<4>[   49.037352] 52ac  00000000 00000000 00000000 00000001 00000000 c0bb52c0 c0bb52c0 00000000
<4>[   49.037381] 52cc  00000000 00000000 00000000 00000001 00000000 c0bb52e0 c0bb52e0 00000000
<4>[   49.037409] 52ec  00000000 00000000 00000000 00000001 00000000 c0bb5300 c0bb5300 00000000
<4>[   49.037438]
<4>[   49.037438] R9: 0xd160e4a0:
<4>[   49.037444] e4a0  d11eb39c d160e0dc d127ade8 eabd7e24 d1221200 eabd7e00 00000000 00000000
<4>[   49.037473] e4c0  00000000 00000000 00000000 00000000 00000018 00000028 00000000 00000000
<4>[   49.037502] e4e0  ffffffff 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.037530] e500  d160e520 00000000 00000000 edae8000 00000000 c0243048 00000000 00000000
<4>[   49.037558] e520  00000002 00000020 b6679000 00000000 00000000 00000000 00000000 00000000
<4>[   49.037587] e540  00000000 00000000 00000000 00000000 00000018 00000028 00000000 00000000
<4>[   49.037616] e560  ffffffff 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<4>[   49.037645] e580  ed8d7600 cd6dccc0 ed8d76a0 d127afc8 d160e8d0 d160ecd0 00000000 00000000
<0>[   49.037676] Process surfaceflinger (pid: 104, stack limit = 0xedb40210)
<0>[   49.037682] Stack: (0xedb41cb0 to 0xedb42000)
<0>[   49.037690] 1ca0:                                     024000c0 c049e130 edb41cdc edb41cc8
<0>[   49.037699] 1cc0: c0160d28 c01eabe8 ee801e80 c0bb528c edb41d1c d53dbb80 ee1dec00 d53dbb80
<0>[   49.037708] 1ce0: 00000001 c0bb528c d53db300 d53db380 edb41d14 edb41d00 c045cbc0 c045c1f0
<0>[   49.037716] 1d00: c045cba0 00000001 edb41d64 edb41d18 c04832f0 c045cbac edb41d3c edb41d28
<0>[   49.037724] 1d20: c0160d28 c01eabe8 ee801f00 00076f08 edb41d7c ee1dec00 c0244aa0 d53dbb80
<0>[   49.037732] 1d40: ee1dec00 d1048800 00000001 ee1ded00 edae8000 d53dbb80 edb41d7c edb41d68
<0>[   49.037741] 1d60: c048376c c0483158 b6679000 00000000 edb41e2c edb41d80 c04848b4 c0483758
<0>[   49.037749] 1d80: 00000000 00000000 00000000 00000001 b6af68c0 b6add320 b6aca3f0 ee29c030
<0>[   49.037757] 1da0: ee1ded00 edae8000 00000018 b6aca408 00000000 00000000 00000002 00000003
<0>[   49.037765] 1dc0: ee1dec00 c0243048 c0ba6258 edb41e58 3231564e 00000000 00000000 00000000
<0>[   49.037773] 1de0: 00000001 eda95b00 00000054 00000003 00000000 ee29c028 ee1df828 00000000
<0>[   49.037781] 1e00: edb41e2c c0bb528c 00000051 ee1dec00 edb41e58 000000bc edae8000 c079a47c
<0>[   49.037790] 1e20: edb41f04 edb41e30 c0468210 c0484568 00020257 00000000 ffffffff 00000001
<0>[   49.037798] 1e40: edb41e58 c048455c befc85e0 00000000 00000038 c03864bc 00000201 00000002
<0>[   49.037806] 1e60: b6aca408 00000000 b6aca3f0 00000000 b6add320 00000000 b6af68c0 00000000
<0>[   49.037814] 1e80: 00000000 00000000 b6679000 00000000 00000000 00000000 00000000 00000000
<0>[   49.037821] 1ea0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
<0>[   49.037829] 1ec0: 69746572 665f6572 00000000 00000000 00000000 000000bc 00000073 edae8e41
<0>[   49.037837] 1ee0: edae8e40 00000013 c03864bc 00000000 befc85e0 eda6f920 edb41f7c edb41f08
<0>[   49.037846] 1f00: c025e2d8 c0467ff4 edb41f20 00000053 edb41f44 ee0a9490 ed4c3f68 edae64bc
<0>[   49.037854] 1f20: edb41f0b edb41f14 c0268f78 c018ea50 edadb800 c0b4ad3c c0b4872c befc85e0
<0>[   49.037862] 1f40: c03864bc edae8e40 edb41f7c edb41f58 c036a3e4 edae8e41 edae8e40 befc85e0
<0>[   49.037869] 1f60: c03864bc 00000000 00000013 00000000 edb41fa4 edb41f80 c025e590 c025de2c
<0>[   49.037878] 1f80: b4fd5100 00000013 c03864bc 00000036 c01080e4 edb40000 00000000 edb41fa8
<0>[   49.037886] 1fa0: c0107f40 c025e520 b4fd5100 00000013 00000013 c03864bc befc85e0 befc85b0
<0>[   49.037894] 1fc0: b4fd5100 00000013 c03864bc 00000036 b6aca408 00000013 00000000 00000023
<0>[   49.037903] 1fe0: befc85e0 befc85a0 b6dde8a5 b6dd7e28 600f0010 00000013 00000000 00000000
<4>[   49.037917] [<c045c2f0>] (drm_atomic_helper_check_modeset) from [<c045cbc0>] (drm_atomic_helper_check+0x20/0x38)
<4>[   49.037930] [<c045cbc0>] (drm_atomic_helper_check) from [<c04832f0>] (drm_atomic_check_only+0x1a4/0x58c)
<4>[   49.037943] [<c04832f0>] (drm_atomic_check_only) from [<c048376c>] (drm_atomic_async_commit+0x20/0x74)
<4>[   49.037954] [<c048376c>] (drm_atomic_async_commit) from [<c04848b4>] (drm_mode_atomic_ioctl+0x358/0x7b8)
<4>[   49.037965] [<c04848b4>] (drm_mode_atomic_ioctl) from [<c0468210>] (drm_ioctl+0x228/0x4cc)
<4>[   49.037977] [<c0468210>] (drm_ioctl) from [<c025e2d8>] (do_vfs_ioctl+0x4b8/0x6f4)
<4>[   49.037988] [<c025e2d8>] (do_vfs_ioctl) from [<c025e590>] (SyS_ioctl+0x7c/0x8c)
<4>[   49.038000] [<c025e590>] (SyS_ioctl) from [<c0107f40>] (ret_fast_syscall+0x0/0x1c)
<0>[   49.038010] Code: e50b1030 e793a106 1a0001dd e59922bc (e59a3004)

------------------------------------------

Notice that the "PC is at drm_atomic_helper_check_modeset+0x10c"

We create objdump using the following command :-

arm-linux-androideabi-objdump -S out/target/product/gminfo3/obj/KERNEL_OBJ/vmlinux > vmlinux_objdump-new.txt

And its obvious that you build kernel with KALLSYMS and DEBUG_INFO configs.

Now from the OBJDUMP, we get :-

<- The function - drm_atomic_helper_check_modeset  ->

c055c304 <drm_atomic_helper_check_modeset>:

* Zero for success or -errno

*/

int

drm_atomic_helper_check_modeset(struct drm_device *dev,

                                                            struct drm_atomic_state *state)

{

c055c304:           e1a0c00d            mov       ip, sp

c055c308:           e92ddff0             push       {r4, r5, r6, r7, r8, r9, sl, fp, ip, lr, pc}

c055c30c:            e24cb004            sub         fp, ip, #4

c055c310:           e24dd024           sub         sp, sp, #36           ; 0x24

c055c314:           e52de004           push       {lr}                         ; (str lr, [sp, #-4]!)

c055c318:           ebf2cb45             bl            c020f034 <__gnu_mcount_nc>

               struct drm_crtc_state *crtc_state;

               struct drm_connector *connector;

Now , we found the (c055c304 + offset) which is 0xc055c410

<- We found this from the objdump ->

               DRM_DEBUG_ATOMIC("Updating routing for [CONNECTOR:%d:%s]\n",

c055c408:           1a0001dd           bne        c055cb84 <drm_atomic_helper_check_modeset+0x880>

                                             connector->base.id,

                                             connector->name);

               if (connector->state->crtc != connector_state->crtc) {

c055c40c:            e59922bc            ldr          r2, [r9, #700]      ; 0x2bc

c055c410:           e59a3004            ldr          r3, [sl, #4]   ------------------> Point of crash.

c055c414:           e5920004           ldr          r0, [r2, #4]

c055c418:           e1500003           cmp       r0, r3

Looks like r0 and r3 is compared here (connector->state->crtc != connector_state->crtc)

From our original crash logs -

<4>[   49.034695] r10: 00000000  r9 : d160e520  r8 : c0bb528c

<4>[   49.034707] r3 : d160e300  r2 : 00000000  r1 : c0bb528c  r0 : 00000001

Possibilities :-

1. Note r2 is zero here.Hence looks like "connector->state->crtc" has crashed as the state variable is NULL.The offset 0x700 is actually offset of connector->state.However as r9 is holding a valid value, we dont know why r2 is ZERO. 

2. Crash could be on connector_state->crtc which is point of crash. Here s1 may be invalid. But r3 here is not Zero hence a NULL pointer crash cannot be explained.

Just for Note :-

the ldr instruction using the s1 below.

               DRM_DEBUG_ATOMIC("Updating routing for [CONNECTOR:%d:%s]\n",

c055c3f8:            e34c10ec            movt      r1, #49388          ; 0xc0ec

c055c3fc:            e3120010           tst          r2, #16

c055c400:           e50b1030           str          r1, [fp, #-48]       ; 0xffffffd0

               struct drm_connector_state *connector_state;

               struct drm_crtc_state *crtc_state;

               int idx, ret;

               connector = state->connectors[conn_idx];

               connector_state = state->connector_states[conn_idx];

c055c404:           e793a106            ldr          sl, [r3, r6, lsl #2]

FINAL VERDICT :-

WE HAVE MESSED UP SOMEWHERE IN THE BELOW CODE :-

if (connector->state->crtc != connector_state->crtc) {   <-  connector->state may be either NULL or a dangling pointer. This is where crash has happened.

Shall we call this Dynamic tasklets ?

There is a typical problem to be solved as mentioned in the below small note:-

 To solve this problem, a solution is provided using dynamically allocated tasklet as below and explanation is provided as necessary with the code.

>
> -------------------------------------------------------------------------
> static void t_fun(unsigned long t_arg)
> {
>         struct my_dat *data = (struct my_dat *)t_arg;
>         atomic_inc(&counter_bh);
>         pr_info("In BH: counter_th = %d, counter_bh = %d, jiffies=%ld, %ld\n",
>                 atomic_read(&counter_th), atomic_read(&counter_bh),
>                 data->jiffies, jiffies);
>         kfree(data);
> }
>
> static irqreturn_t my_interrupt(int irq, void *dev_id)
> {
>         struct tasklet_struct *t;
>         struct my_dat *data;
>
>         data = (struct my_dat *)kmalloc(sizeof(struct my_dat), GFP_ATOMIC);
>         t = &data->tsk;
>         data->jiffies = jiffies;
>
>         tasklet_init(t, t_fun, (unsigned long)data);
>
>         atomic_inc(&counter_th);
>         tasklet_schedule(t);
>         mdelay(delay);          /* hoke up a delay to try to cause pileup */
>         return IRQ_NONE;        /* we return IRQ_NONE because we are just observing */
> }
>

This part of code initializes a new tasklet everytime on the IRQ. Hence each time the processor services the IRQ there is a new  tasklet is getting created however the function remains the same. Note:- In traditional drivers usually a single tasklet is used and scheduled multiple times using the IRQ

> module_init(my_generic_init);
> module_exit(my_generic_exit);
>
> -------------------------------------------------------------------------
>
> Above solution will not miss any bottom halves.
> Can somebody explains me why above solution is working ?
> What is so special about dynamic tasklets ?

The  solution works because the new tasklet is created everytime with the same callback function. Note that the function should be fully re-entrant for such use-cases which is done using the local variable - arg/data in this case.

Reference :- http://lists.kernelnewbies.org/pipermail/kernelnewbies/2018-June/019197.html

C Language : Write you own sizeof operator and container_of macro

#include<stdio.h>

/* #define SIZEOF(ptr)   ((char *)(&ptr+1) - (char *)(&ptr)) */

#define SIZEOF(t)  ((char *)(((typeof(t) *)0) + 1) - (char *)0)

int main()
{
        int val =1;

        printf("SIZEOF is %d\n", SIZEOF(int));

        return 0;
}

Note the typeof operator. A non-standard operator generally supported by gcc compilers.This is part of gcc compiler extension. Also this programs works for all cases like sizeof(int). I dont think this program can be written using standard C keywords.

The container_of macro :-

#define container_of(ptr, type, member) ({                      \
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
        (type *)( (char *)__mptr - offsetof(type,member) );})

(type *)( (char *)__mptr - offsetof(type,member) );})

A patch to enable EarlyPrintk in Linux Kernel

Below is the patch to enable earlyprintk in TI platforms.

Note that this is only for a present system that I am working

right now and may not apply/work straightaway in all platfrom.

Hence you may have to put effort to port this patch in your 

system.

diff --git a/arch/arm/configs/xyz-ed0-dbg_defconfig b/arch/arm/configs/xyz-ed0-dbg_defconfig

index 688316b..f2fed80 100644

--- a/arch/arm/configs/xyz-ed0-dbg_defconfig

+++ b/arch/arm/configs/xyz-ed0-dbg_defconfig

@@ -181,10 +181,17 @@ CONFIG_HAVE_ARCH_KGDB=y

# CONFIG_STRICT_DEVMEM is not set

CONFIG_ARM_UNWIND=y

# CONFIG_DEBUG_USER is not set

-# CONFIG_DEBUG_LL is not set

-CONFIG_DEBUG_LL_INCLUDE="mach/debug-macro.S"

+CONFIG_DEBUG_LL=y

+CONFIG_DEBUG_LL_INCLUDE="debug/8250.S"

# CONFIG_DEBUG_UART_8250 is not set

+CONFIG_DEBUG_LL_UART_8250=y

+CONFIG_DEBUG_UART_PHYS=0x48020000

+CONFIG_DEBUG_UART_VIRT=0xFA020000

+CONFIG_DEBUG_UART_8250_SHIFT=2

CONFIG_UNCOMPRESS_INCLUDE="debug/uncompress.h"

+CONFIG_DEBUG_UNCOMPRESS=y

+CONFIG_EARLY_PRINTK=y

+CONFIG_EARLY_PRINTK_DIRECT=y

# CONFIG_PID_IN_CONTEXTIDR is not set

# CONFIG_DEBUG_SET_MODULE_RONX is not set

# CONFIG_CORESIGHT is not set

diff --git a/arch/arm/mach-omap2/omap_hwmod_7xx_data.c b/arch/arm/mach-omap2/omap_hwmod_7xx_data.c

index cc48abb..34331e9 100755

--- a/arch/arm/mach-omap2/omap_hwmod_7xx_data.c

+++ b/arch/arm/mach-omap2/omap_hwmod_7xx_data.c

@@ -2950,7 +2950,8 @@ static struct omap_hwmod dra7xx_uart3_hwmod = {

    .class          = &dra7xx_uart_hwmod_class,

    .clkdm_name     = "l4per_clkdm",

    .main_clk  = "uart3_gfclk_mux",

-    .flags          = HWMOD_SWSUP_SIDLE_ACT | DEBUG_OMAP4UART3_FLAGS,

+    .flags          = HWMOD_SWSUP_SIDLE_ACT | DEBUG_OMAP4UART3_FLAGS |

+                    DEBUG_OMAPUART_FLAGS,

    .prcm = {

          .omap4 = {

               .clkctrl_offs = DRA7XX_CM_L4PER_UART3_CLKCTRL_OFFSET,

Reboot testing script for Linux

In many scenarios, we come up with issue during power cycle or reboot of an embedded devices.
As you know it may not be possible to do multiple power cycle manually, here is a teraterm script 
which can be named as "AutoReboot_string_boot_done.ttl" and loaded as a MACRO in teraterm for linux devices. Go to Control->Macros and load the file.

The content of "AutoReboot_string_boot_done.ttl" is as below :-

; TTL Script used for automatic reboot test

username = 'root'
password = 'root'
usernamePrompt = 'login'
passwordPrompt = 'Password'
oopsPrompt = 'Oops'
panicPrompt = 'Kernel panic'
;mountPrompt = 'BOOT-TIME| PowerModing HMIReady'
mountPrompt = 'U-Boot'
maxLoop = 200000

gettime now '%Y/%m/%d-%H:%M:%S'#13#10
sprintf2 startTime 'AutoReboot started at %s' now

gettime logfile "D:/AutoReboot-test-log-%Y%m%d-%H%M%S.txt"
logopen logfile 0 0 0 0 0 0
logwrite startTime

for i 1 maxLoop
  wait mountPrompt
  if result = 1 then
    pause 40
    sendln
    gettime now '%Y/%m/%d-%H:%M:%S'
    sprintf2 rebootTime 'AutoReboot %d times at %s'#13#10 i now
    dispstr rebootTime
    logwrite rebootTime
    sendln
    sendln 'reboot'
  endif
next

gettime now '%Y/%m/%d-%H:%M:%S'
sprintf2 stopTime 'AutoReboot stopped at %s after %d times reboot'#13#10 now i
logwrite stopTime

logclose

-------------

How do the script work?

The above script issue a reboot command in the console after 40second of  mountPrompt = 'U-Boot'.Which means once U-Boot string comes in console logs the script waits for 40seconds for issuing the reboot command. The script can be customized as much as possible.

Loop Back Testing for UART

For fundamental UART related tutorial, please refer - http://www.circuitbasics.com/basics-uart-communication/

Scope of Article - UART loop back test from device driver side on OMAP platform.

https://github.com/nsekhar/serialcheck

About serialcheck/serialstatus : These are open-source userspace linux utility which are developed by TI for validating the serial ports on OMAP platform. This comes under - GNU GENERAL PUBLIC LICENSE 

What is a LOOP Back test?

A loopback test can be used to test/debug UART serial communications. It can show problems in the serial port, the cable, or the software. In this test normally the RX/TX and RTS/CTS pins are shorted on the same UART.  

A loopback test on OMAP platform with Internal Loop back via programming the registers and using the Hardware flow control didn't work properly in our Board. We found that data overflow using the serial check when such kind of test was performed. 

However We tried another approach where we will SHORT the RX/TX tiles and also the flow control line (RTS/CTS) and run serialcheck.

What is RTS / CTS Hardware Flow Control? RTS / CTS Flow Control is another flow control mechanism that is part of the RS232 standard.

It makes use of two further pins on the RS232 connector, RTS (Request to Send) and CTS (Clear to Send). These two lines allow the receiver and the transmitter to alert each other to their state. A transmitter raises its RTS line, which causes an interrupt on the receiver, i.e - hey can I send some data?. If the receiver is in a position to receive the data it will assert its CTS line, i.e - yes you can start sending. The raising and lowering of these lines allows device drivers which implement hardware flow control code to maintain a reliable data connection between transmitter and receiver. The advantage to this approach is that damaging data in transit does not effect the flow control mechanism (as in XOn / XOff).

C programming : Finding out Endian-ness of Machine.

int main( )
{
          int num = 1;
 
          if(*(char *)&num == 1)
          {
                  printf("\nLittle-Endian\n");
          }
          else
         {
                  printf("Big-Endian\n");
         }

}