WARNING: CPU: 2 PID: 42 at drivers/gpu/drm/drm_modeset_lock.c:276

[Stefan Wahren]

Hi Maíra,

Am 28.12.22 um 20:49 schrieb Maíra Canal:
> Hi Stefan,
>
> I was able to reproduce this error on drm-misc-next. I bisected,
> and I got into commit 6bed2ea3cb38. I noticed that the crtc->mutex is
> being locked twice, and this might be causing the problem. I wrote a
> patch to try to fix this issue, and after applying the patch, I wasn't
> able to reproduce the error anymore.
>
> Let me know if you were able to reproduce the warning after applying 
> this patch.

the patch works as expected and avoid the warning. I tested it on top of 
v6.1 with RPi 3 B+ and RPi 4 B.

In case you send the patch please add the Fixes tag so the patch get 
backported to stable.

Thanks a lot

Stefan

>
> Best Regards,
> - Maíra Canal
>
> ---
>
> From f6c910327d060e2314947e7e456db363a6164a49 Mon Sep 17 00:00:00 2001
> From: =?UTF-8?q?Ma=C3=ADra=20Canal?= <mcanal at igalia.com>
> Date: Wed, 28 Dec 2022 16:14:34 -0300
> Subject: [PATCH] drm/vc4: don't lock crtc's mutex on reset link
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
>
> Current, crtc->mutex is being locked twice: first, by
> vc4_hdmi_reset_link() and them, internally, by
> drm_atomic_get_crtc_state(). This produces the following output when
> PROVE_LOCKING is enabled:
>
> [  825.612809] ------------[ cut here ]------------
> [  825.612852] WARNING: CPU: 1 PID: 116 at 
> drivers/gpu/drm/drm_modeset_lock.c:276 
> drm_modeset_drop_locks+0x60/0x68 [drm]
> [  825.613458] Modules linked in: 8021q mrp garp stp llc
> raspberrypi_cpufreq brcmfmac brcmutil crct10dif_ce hci_uart cfg80211
> btqca btbcm bluetooth vc4 raspberrypi_hwmon snd_soc_hdmi_codec cec
> clk_raspberrypi ecdh_generic drm_display_helper ecc rfkill
> drm_dma_helper drm_kms_helper pwm_bcm2835 bcm2835_thermal bcm2835_rng
> rng_core i2c_bcm2835 drm fuse ip_tables x_tables ipv6
> [  825.613735] CPU: 1 PID: 116 Comm: kworker/1:2 Tainted: G W 
> 6.1.0-rc6-01399-g941aae326315 #3
> [  825.613759] Hardware name: Raspberry Pi 3 Model B Rev 1.2 (DT)
> [  825.613777] Workqueue: events output_poll_execute [drm_kms_helper]
> [  825.614038] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS 
> BTYPE=--)
> [  825.614063] pc : drm_modeset_drop_locks+0x60/0x68 [drm]
> [  825.614603] lr : drm_helper_probe_detect+0x120/0x1b4 [drm_kms_helper]
> [  825.614829] sp : ffff800008313bf0
> [  825.614844] x29: ffff800008313bf0 x28: ffffcd7778b8b000 x27: 
> 0000000000000000
> [  825.614883] x26: 0000000000000001 x25: 0000000000000001 x24: 
> ffff677cc35c2758
> [  825.614920] x23: ffffcd7707d01430 x22: ffffcd7707c3edc7 x21: 
> 0000000000000001
> [  825.614958] x20: 0000000000000000 x19: ffff800008313c10 x18: 
> 000000000000b6d3
> [  825.614995] x17: ffffcd777835e214 x16: ffffcd7777cef870 x15: 
> fffff81000000000
> [  825.615033] x14: 0000000000000000 x13: 0000000000000099 x12: 
> 0000000000000002
> [  825.615070] x11: 72917988020af800 x10: 72917988020af800 x9 : 
> 72917988020af800
> [  825.615108] x8 : ffff677cc665e0a8 x7 : d00a8c180000110c x6 : 
> ffffcd77774c0054
> [  825.615145] x5 : 0000000000000000 x4 : 0000000000000001 x3 : 
> 0000000000000000
> [  825.615181] x2 : ffff677cc55e1880 x1 : ffffcd7777cef8ec x0 : 
> ffff800008313c10
> [  825.615219] Call trace:
> [  825.615232]  drm_modeset_drop_locks+0x60/0x68 [drm]
> [  825.615773]  drm_helper_probe_detect+0x120/0x1b4 [drm_kms_helper]
> [  825.616003]  output_poll_execute+0xe4/0x224 [drm_kms_helper]
> [  825.616233]  process_one_work+0x2b4/0x618
> [  825.616264]  worker_thread+0x24c/0x464
> [  825.616288]  kthread+0xec/0x110
> [  825.616310]  ret_from_fork+0x10/0x20
> [  825.616335] irq event stamp: 7634
> [  825.616349] hardirqs last  enabled at (7633): [<ffffcd777831ee90>] 
> _raw_spin_unlock_irq+0x3c/0x78
> [  825.616384] hardirqs last disabled at (7634): [<ffffcd7778315a78>] 
> __schedule+0x134/0x9f0
> [  825.616411] softirqs last  enabled at (7630): [<ffffcd7707aacea0>] 
> local_bh_enable+0x4/0x30 [ipv6]
> [  825.617019] softirqs last disabled at (7618): [<ffffcd7707aace70>] 
> local_bh_disable+0x4/0x30 [ipv6]
> [  825.617586] ---[ end trace 0000000000000000 ]---
>
> So, don't lock crtc->mutex inside the driver and let the right lock be
> automatically acquired by drm_atomic_get_crtc_state().
>
> Reported-by: Stefan Wahren <stefan.wahren at i2se.com>
> Signed-off-by: Maíra Canal <mcanal at igalia.com>
> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 4 ----
>  1 file changed, 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c 
> b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index 0d3313c71f2f..b3b1958b5f4d 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -340,10 +340,6 @@ static int vc4_hdmi_reset_link(struct 
> drm_connector *connector,
>      if (!crtc)
>          return 0;
>
> -    ret = drm_modeset_lock(&crtc->mutex, ctx);
> -    if (ret)
> -        return ret;
> -
>      crtc_state = crtc->state;
>      if (!crtc_state->active)
>          return 0;