[PATCH] drm: Try to acquire modeset lock on panic or sysrq

[Daniel Vetter]

On Tue, Mar 25, 2014 at 01:25:15PM +0100, Thierry Reding wrote:
> From: Thierry Reding <treding at nvidia.com>
> 
> Commit 62ff94a54921 "drm/crtc-helper: remove LOCKING from kerneldoc"
> causes drm_helper_crtc_in_use() and drm_helper_encoder_in_use() to
> complain loudly during a kernel panic or sysrq processing. This is
> caused by nobody acquiring the modeset lock in these code paths.
> 
> This patch fixes this by trying to acquire the modeset lock for each
> FB helper that's forced to kernel mode. If the lock can't be acquired,
> it's likely that somebody else is performing a modeset. However, doing
> another modeset concurrently might make things even worse, so the safe
> option is to simply bail out in that case.
> 
> Signed-off-by: Thierry Reding <treding at nvidia.com>

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

I guess we should also try to grab the crtc locks, but blocking ongoing
pageflips shouldn't be too harmful. Otherwise I think this should help a
bit in some of the hilarious failures I've seen when kms itself blows up.
-Daniel

> ---
>  drivers/gpu/drm/drm_fb_helper.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 9580393628a0..afce7cbd5149 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -325,12 +325,21 @@ static bool drm_fb_helper_force_kernel_mode(void)
>  		return false;
>  
>  	list_for_each_entry(helper, &kernel_fb_helper_list, kernel_fb_list) {
> -		if (helper->dev->switch_power_state == DRM_SWITCH_POWER_OFF)
> +		struct drm_device *dev = helper->dev;
> +
> +		if (dev->switch_power_state == DRM_SWITCH_POWER_OFF)
> +			continue;
> +
> +		if (!mutex_trylock(&dev->mode_config.mutex)) {
> +			error = true;
>  			continue;
> +		}
>  
>  		ret = drm_fb_helper_restore_fbdev_mode(helper);
>  		if (ret)
>  			error = true;
> +
> +		mutex_unlock(&dev->mode_config.mutex);
>  	}
>  	return error;
>  }
> -- 
> 1.9.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch