Kernel warning when mode setting with no connectors connected

[Daniel Vetter]

On Sat, Apr 12, 2014 at 12:32 PM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
>
> I've been seeing the following warning when a mode is attempted to be
> set after boot when no display connectors are connected.  This typically
> happens if your platform has just a HDMI output, and the TV connected
> to the HDMI output is fully powered off.
>
> This has been happening for quite some time now... I should've probably
> reported it earlier.  I've seen it both with imx-drm and armada-drm.
>
> WARNING: CPU: 0 PID: 846 at drivers/gpu/drm/drm_crtc.c:2074 drm_mode_set_config_internal+0xd0/0xd8()
> Modules linked in: rfcomm bnep bluetooth 6lowpan_iphc brcmfmac brcmutil snd_soc_fsl_spdif imx_pcm_dma imx_sdma imx_thermal imx2_wdt snd_soc_imx_spdif nfsd exportfs
> CPU: 0 PID: 846 Comm: Xorg Not tainted 3.14.0+ #1027
> Backtrace:
> [<c0011cf4>] (dump_backtrace) from [<c0011e94>] (show_stack+0x18/0x1c)
>  r6:0000081a r5:00000009 r4:00000000 r3:00000000
> [<c0011e7c>] (show_stack) from [<c066b4e8>] (dump_stack+0x74/0x90)
> [<c066b474>] (dump_stack) from [<c0024588>] (warn_slowpath_common+0x70/0x90)
>  r4:00000000 r3:c09344b8
> [<c0024518>] (warn_slowpath_common) from [<c00245cc>] (warn_slowpath_null+0x24/0x2c)
>  r8:ea3c3818 r7:00000001 r6:e9904500 r5:00000000 r4:ea3c3818
> [<c00245a8>] (warn_slowpath_null) from [<c03474e4>] (drm_mode_set_config_internal+0xd0/0xd8)
> [<c0347414>] (drm_mode_set_config_internal) from [<c03497c8>] (drm_mode_setcrtc+0x378/0x4b4)
>  r6:ea3c3400 r5:e8d78b00 r4:e9981e48 r3:00000000
> [<c0349450>] (drm_mode_setcrtc) from [<c033c9d0>] (drm_ioctl+0x3f4/0x498)
>  r10:c06b8fa0 r9:e9981e48 r8:e9981e48 r7:e9980000 r6:e98260c0 r5:000000a2
>  r4:ea3c3400
> [<c033c5dc>] (drm_ioctl) from [<c01095b8>] (do_vfs_ioctl+0x80/0x5d4)
>  r10:00000000 r9:e9980000 r8:00000006 r7:c06864a2 r6:be911850 r5:e9826900
>  r4:ea3bf100
> [<c0109538>] (do_vfs_ioctl) from [<c0109b4c>] (SyS_ioctl+0x40/0x5c)
>  r9:e9980000 r8:00000006 r7:c06864a2 r6:be911850 r5:e9826900 r4:e9826900
> [<c0109b0c>] (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30)
>  r8:c000e864 r7:00000036 r6:00000006 r5:c06864a2 r4:be911850 r3:00000001
> ---[ end trace 46155b08f61d1185 ]---
>
> In drm_mode_set_config_internal(), this check is done:
>
>         fb = set->fb;
>
>         ret = crtc->funcs->set_config(set);
>         if (ret == 0) {
>                 /* crtc->fb must be updated by ->set_config, enforces this. */
>                 WARN_ON(fb != crtc->fb);
>         }
>
> However, drm_crtc_helper_set_config() does this:
>
>         if (!set->mode)
>                 set->fb = NULL;
>
>         if (set->fb) {
> ...
>         } else {
>                 DRM_DEBUG_KMS("[CRTC:%d] [NOFB]\n", set->crtc->base.id);
>                 return drm_crtc_helper_disable(set->crtc);
>         }
>
> which results in a zero return with crtc->fb set to NULL, possibly
> being different to what set->fb was originally.
>
> I haven't debugged this fully yet as the only side effect is the
> warning.

This is strange - passing a NULL mode and a non-NULL fb to
->set_config is a bug, and in the i915 ->set_config callback
(intel_crtc_set_config) we even have a BUG_ON to check for that. A few
release ago the fb helper code violated this which wreak a bit of
havoc in the i915 modeset code until I've fixed it, but now this
really should hold everywhere.

Also I've read through the setcrtc ioctl and we only grab an fb if the
mode is valid. So tbh I have no idea how things blew up like you
describe. In e58de88078e92 I even added the checks we have in i915 to
the crtc helper library. I'm confused.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch