[PATCH] drm/atomic: don't reset link-status to GOOD without ALLOW_MODESET

[Daniel Vetter]

On Wed, Jun 03, 2020 at 10:45:23AM +0000, Simon Ser wrote:
> In update_output_state, the link-status property was reset to GOOD to
> ensure legacy drmModeSetCrtc re-trains the link. However this auto-reset
> is also performed on an atomic commit without ALLLOW_MODESET. If a
> driver reads link-status to figure out whether to re-train the link,
> this could cause an atomic commit failure. User-space doesn't expect
> such a failure, because commits without ALLOW_MODESET aren't supposed to
> fail because of link training issues.
> 
> Change update_output_state to implicitly reset link-status to GOOD only
> if ALLOW_MODESET is set. This is the case for legacy drmModeSetCrtc
> because drm_atomic_state_init sets it (and is used in
> drm_atomic_helper_set_config, called from drm_mode_setcrtc).
> 
> Drivers don't seem to read link-status at the moment -- they seem to
> rely on user-space performing a modeset instead. So this shouldn't
> result in any change in behaviour, this should only prevent future
> failures if drivers start reading link-status.
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Suggested-by: Pekka Paalanen <ppaalanen at gmail.com>

I think we should have Cc: stable at vger.kernel.org on this. Also I think
would be really awesome if we can update/extend the igt, but I guess if
you don't have a chamelium it's a bit hard to make that happen :-/

Also I guess you'll reflect this in your doc patch?

With the cc: stable this has my Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Cheers, Daniel

> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Manasi Navare <manasi.d.navare at intel.com>
> ---
>  drivers/gpu/drm/drm_atomic.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index 965173fd0ac2..3d9d9e6f7397 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -1426,7 +1426,8 @@ static int update_output_state(struct drm_atomic_state *state,
>  				return ret;
>  
>  			/* Make sure legacy setCrtc always re-trains */
> -			new_conn_state->link_status = DRM_LINK_STATUS_GOOD;
> +			if (state->allow_modeset)
> +				new_conn_state->link_status = DRM_LINK_STATUS_GOOD;
>  		}
>  	}
>  
> -- 
> 2.26.2
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch