Re: [PATCH] drm: Schedule the output_poll_work with 1s delay if we have delayed event

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jan 04, 2017 at 02:00:53PM +0200, Peter Ujfalusi wrote:
> Instead of scheduling the work to handle the initial delayed event, use 1s
> delay.
> 
> When the delayed event is handled w/o delay - in a similar matter when the
> poll had been initialized before drm_helper_probe_single_connector_modes()
> is called - it triggers a race in Optimus setups.
> 
> Fixes: 339fd36238dd ("drm: drm_probe_helper: Fix output_poll_work scheduling")
> Cc: stable@vger.kernel.org   # v4.9
> Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
> ---
> Hi,
> 
> related bug report: https://bugs.freedesktop.org/show_bug.cgi?id=98690
> 
> Regards,
> Peter
> 
>  drivers/gpu/drm/drm_probe_helper.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
> index 98ed110e28ed..f30c14b0a72f 100644
> --- a/drivers/gpu/drm/drm_probe_helper.c
> +++ b/drivers/gpu/drm/drm_probe_helper.c
> @@ -146,8 +146,9 @@ void drm_kms_helper_poll_enable_locked(struct drm_device *dev)
>  	drm_connector_list_iter_put(&conn_iter);
>  
>  	if (dev->mode_config.delayed_event) {
> +		/* Use short (1s) delay to handle the initial delayed event */
>  		poll = true;
> -		delay = 0;
> +		delay = HZ;

This smells a bit like duct-tape papering over some other race. Lack of
locking or something else. Most likely some drivers enable polling a bit
too early in their load sequence. And if we can't figure this out, then we
need some really big FIXME: here that this papers over driver races.
-Daniel

>  	}
>  
>  	if (poll)
> -- 
> 2.11.0
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch