On Sun, Dec 23, 2018 at 02:55:52PM +0100, Noralf Trønnes wrote: > > > Den 23.12.2018 01.55, skrev Peter Wu: > > After drm_fb_helper_fbdev_setup calls drm_fb_helper_init, > > "dev->fb_helper" will be initialized (and thus drm_fb_helper_fini will > > have some effect). After that, drm_fb_helper_initial_config is called > > which may call the "fb_probe" driver callback. > > > > This driver callback may call drm_fb_helper_defio_init (as is done by > > drm_fb_helper_generic_probe) or set a framebuffer (as is done by bochs) > > as documented. These are normally cleaned up on exit by > > drm_fb_helper_fbdev_teardown which also calls drm_fb_helper_fini. > > > > If an error occurs after "fb_probe", but before setup is complete, then > > calling just drm_fb_helper_fini will leak resources. This was triggered > > by df2052cc922 ("bochs: convert to drm_fb_helper_fbdev_setup/teardown"): > > > > [ 50.008030] bochsdrmfb: enable CONFIG_FB_LITTLE_ENDIAN to support this framebuffer > > [ 50.009436] bochs-drm 0000:00:02.0: [drm:drm_fb_helper_fbdev_setup] *ERROR* fbdev: Failed to set configuration (ret=-38) > > [ 50.011456] [drm] Initialized bochs-drm 1.0.0 20130925 for 0000:00:02.0 on minor 2 > > [ 50.013604] WARNING: CPU: 1 PID: 1 at drivers/gpu/drm/drm_mode_config.c:477 drm_mode_config_cleanup+0x280/0x2a0 > > [ 50.016175] CPU: 1 PID: 1 Comm: swapper/0 Tainted: G T 4.20.0-rc7 #1 > > [ 50.017732] EIP: drm_mode_config_cleanup+0x280/0x2a0 > > ... > > [ 50.023155] Call Trace: > > [ 50.023155] ? bochs_kms_fini+0x1e/0x30 > > [ 50.023155] ? bochs_unload+0x18/0x40 > > > > This can be reproduced with QEMU and CONFIG_FB_LITTLE_ENDIAN=n. > > > > Link: https://lkml.kernel.org/r/20181221083226.GI23332@shao2-debian > > Link: https://lkml.kernel.org/r/20181223004315.GA11455@al > > Fixes: 8741216396b2 ("drm/fb-helper: Add drm_fb_helper_fbdev_setup/teardown()") > > Reported-by: kernel test robot > > Cc: Noralf Trønnes > > Signed-off-by: Peter Wu > > --- > > drivers/gpu/drm/drm_fb_helper.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c > > index 9d64f874f965..432e0f3b9267 100644 > > --- a/drivers/gpu/drm/drm_fb_helper.c > > +++ b/drivers/gpu/drm/drm_fb_helper.c > > @@ -2860,7 +2860,7 @@ int drm_fb_helper_fbdev_setup(struct drm_device *dev, > > return 0; > > err_drm_fb_helper_fini: > > - drm_fb_helper_fini(fb_helper); > > + drm_fb_helper_fbdev_teardown(dev); > > This change will break the error path for drm_fbdev_generic_setup() > because drm_fb_helper_generic_probe() cleans up on error but doesn't > clear drm_fb_helper->fb resulting in a double drm_framebuffer_remove(). This should probably considered a bug of drm_fb_helper_generic_probe. Ownership of fb_helper should remain with the caller. The caller can detect an error and act accordingly. > My assumption has been that the drm_fb_helper_funcs->fb_probe callback > cleans up its resources on error. Clearly this is not the case for bochs, so > my take on this is that bochsfb_create() needs to clean up on error. That assumption still holds for bochs. The problem is this sequence: - drm_fb_helper_fbdev_setup is called. - fb_probe succeeds (this is crucial). - register_framebuffer fails. - error path of setup is triggered. As fb_helper is fully setup by drivers, the drm_fb_helper core should fully deallocate it again on the error path or else a leak occurs. > Gerd has a patchset that switches bochs over to the generic fbdev > emulation, but ofc that doesn't help with 4.20: > https://patchwork.freedesktop.org/series/54269/ And that does not help with other users of the drm_fb_helper who use functions like drm_fb_helper_defio_init. They will likely run in the same problem. I don't have a way to test tinydrm or other drivers, but if you force register_framebuffer to fail, you should be able to reproduce the problem with drm_fb_helper_generic_probe. -- Kind regards, Peter Wu https://lekensteyn.nl