Hi Mark,
I currently work on the DCU DRM driver (drivers/gpu/drm/fsl-dcu/) on a Linux 4.4 kernel. With CONFIG_LOCKDEP enabled I get the following warning on startup:
[ 1.327284] ------------[ cut here ]------------ [ 1.332010] WARNING: CPU: 0 PID: 1 at kernel/locking/lockdep.c:2755 lockdep_trace_alloc+0x120/0x124() [ 1.341358] DEBUG_LOCKS_WARN_ON(irqs_disabled_flags(flags)) [ 1.346807] Modules linked in: [ 1.350161] CPU: 0 PID: 1 Comm: swapper Not tainted 4.4.0-00013-g7e569bc-dirty #41 [ 1.357851] Hardware name: Freescale Vybrid VF5xx/VF6xx (Device Tree) [ 1.364376] Backtrace: [ 1.366924] [<80013ba0>] (dump_backtrace) from [<80013d98>] (show_stack+0x18/0x1c) [ 1.374609] r7:80054ac4 r6:00000ac3 r5:00000009 r4:00000000 [ 1.380434] [<80013d80>] (show_stack) from [<8028eb84>] (dump_stack+0x24/0x28) [ 1.387795] [<8028eb60>] (dump_stack) from [<80021ecc>] (warn_slowpath_common+0x88/0xb4) [ 1.396021] [<80021e44>] (warn_slowpath_common) from [<80021f30>] (warn_slowpath_fmt+0x38/0x40) [ 1.404842] r8:000001fc r7:80378238 r6:024080c0 r5:8f801f00 r4:808d63fc [ 1.411743] [<80021efc>] (warn_slowpath_fmt) from [<80054ac4>] (lockdep_trace_alloc+0x120/0x124) [ 1.420654] r3:808d98e0 r2:808d63fc [ 1.424325] r4:600000d3 [ 1.426950] [<800549a4>] (lockdep_trace_alloc) from [<800dad44>] (kmem_cache_alloc+0x30/0x110) [ 1.435684] r5:8f801f00 r4:024080c0 [ 1.439380] [<800dad14>] (kmem_cache_alloc) from [<80378238>] (regcache_rbtree_write+0x138/0x504) [ 1.448374] r7:8fa1b5c0 r6:8fa14800 r5:00000218 r4:00000000 [ 1.454196] [<80378100>] (regcache_rbtree_write) from [<803773e0>] (regcache_write+0x5c/0x64) [ 1.462838] r10:8fa1b588 r9:00000001 r8:00000004 r7:00000000 r6:00000000 r5:000001fc [ 1.470882] r4:8fa14800 [ 1.473504] [<80377384>] (regcache_write) from [<80375d20>] (_regmap_raw_write+0x124/0x5fc) [ 1.481972] r7:00000000 r6:8fa1b584 r5:00000004 r4:8fa14800 [ 1.487796] [<80375bfc>] (_regmap_raw_write) from [<8037626c>] (_regmap_bus_raw_write+0x74/0x90) [ 1.496703] r10:00000000 r9:00000024 r8:00000000 r7:8fa14800 r6:00000000 r5:000001fc [ 1.504745] r4:8fa14800 [ 1.507365] [<803761f8>] (_regmap_bus_raw_write) from [<80375368>] (_regmap_write+0x60/0x9c) [ 1.515923] r7:8fa14800 r6:00000000 r5:000001fc r4:8fa14800 [ 1.521744] [<80375308>] (_regmap_write) from [<80376760>] (regmap_write+0x48/0x68) [ 1.529516] r7:000001fc r6:00000000 r5:000001fc r4:8fa14800 [ 1.535348] [<80376718>] (regmap_write) from [<803538e8>] (fsl_dcu_drm_crtc_create+0x98/0xe8) [ 1.543998] r7:000001fc r6:00000220 r5:8f9a6810 r4:00000200 [ 1.549824] [<80353850>] (fsl_dcu_drm_crtc_create) from [<80352df0>] (fsl_dcu_drm_modeset_init+0x5c/0xfc) [ 1.559522] r9:809a25ec r8:8f9a7000 r7:8f8bd410 r6:8f9a6810 r5:8f9a7000 r4:8f9a6810 [ 1.567490] [<80352d94>] (fsl_dcu_drm_modeset_init) from [<80352894>] (fsl_dcu_load+0x34/0x1d8) [ 1.576308] r7:8f8bd410 r6:8f9a6810 r5:8f9a7000 r4:8f9a7000 [ 1.582141] [<80352860>] (fsl_dcu_load) from [<803330bc>] (drm_dev_register+0xb4/0xc4) [ 1.590180] r9:809a25ec r8:8f9a7000 r7:8f8bd400 r6:00000000 r5:00000000 r4:8f9a7000 [ 1.598140] [<80333008>] (drm_dev_register) from [<80352c8c>] (fsl_dcu_drm_probe+0x254/0x2cc) [ 1.606785] r7:8f8bd400 r6:8f9a6810 r5:8f8bd410 r4:00000000 [ 1.612620] [<80352a38>] (fsl_dcu_drm_probe) from [<80362d3c>] (platform_drv_probe+0x58/0xb4) [ 1.621270] r8:00000000 r7:fffffdfb r6:80a0aee8 r5:8f8bd410 r4:ffffffed [ 1.628159] [<80362ce4>] (platform_drv_probe) from [<803611c8>] (driver_probe_device+0x210/0x304) [ 1.637153] r7:80a0aee8 r6:00000000 r5:8f8bd410 r4:81242468 [ 1.642971] [<80360fb8>] (driver_probe_device) from [<80361358>] (__driver_attach+0x9c/0xa0) [ 1.651530] r9:809a25ec r8:000000cd r7:00000000 r6:8f8bd444 r5:80a0aee8 r4:8f8bd410 [ 1.659503] [<803612bc>] (__driver_attach) from [<8035f41c>] (bus_for_each_dev+0x70/0xa4) [ 1.667801] r7:00000000 r6:803612bc r5:80a0aee8 r4:00000000 [ 1.673639] [<8035f3ac>] (bus_for_each_dev) from [<80360c10>] (driver_attach+0x24/0x28) [ 1.681763] r6:80a0b7f0 r5:8fa19580 r4:80a0aee8 [ 1.686517] [<80360bec>] (driver_attach) from [<8036083c>] (bus_add_driver+0x1a8/0x220) [ 1.694657] [<80360694>] (bus_add_driver) from [<80361bfc>] (driver_register+0x80/0x100) [ 1.702869] r7:8fa16a40 r6:809eb3e0 r5:809bc6fc r4:80a0aee8 [ 1.708685] [<80361b7c>] (driver_register) from [<80362c60>] (__platform_driver_register+0x48/0x50) [ 1.717855] r5:809bc6fc r4:80a0b7f0 [ 1.721543] [<80362c18>] (__platform_driver_register) from [<809bc718>] (fsl_dcu_drm_platform_driver_init+0x1c/0x20) [ 1.732202] r5:809bc6fc r4:809eb3e0 [ 1.735894] [<809bc6fc>] (fsl_dcu_drm_platform_driver_init) from [<800096b4>] (do_one_initcall+0x98/0x1e4) [ 1.745689] [<8000961c>] (do_one_initcall) from [<809a2e3c>] (kernel_init_freeable+0x138/0x1dc) [ 1.754510] r10:809d6850 r9:809a25ec r8:000000cd r7:80a40d40 r6:80a40d40 r5:00000006 [ 1.762553] r4:809e5840 [ 1.765175] [<809a2d04>] (kernel_init_freeable) from [<8073ef00>] (kernel_init+0x18/0xf0) [ 1.773468] r10:00000000 r9:00000000 r8:00000000 r7:00000000 r6:00000000 r5:8073eee8 [ 1.781506] r4:80a40d40 [ 1.784128] [<8073eee8>] (kernel_init) from [<8000f9d0>] (ret_from_fork+0x14/0x24) [ 1.791810] r5:8073eee8 r4:00000000 [ 1.795539] ---[ end trace e76a00401296776a ]---
I do use REGCACHE_RBTREE along with regmap_write from within the probe code path of the driver. This ultimately leads to an allocation. However, what I don't understand is why the allocation is leading to that error. The actual allocation happens in regcache_rbtree_node_alloc and seems to be a rather common kzalloc with GFP_KERNEL...
The comment in __lockdep_trace_alloc says: "Oi! Can't be having __GFP_FS allocations with IRQs disabled.".
Not sure if this is a Linux 4.4 issue, Fabio Estevam reported a similar issue just recently, not sure if that is related: https://lkml.org/lkml/2016/1/11/284
-- Stefan