Re: [PATCH 01/13] drm/atomic: Only update crtc->x/y if it's part of the state.

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 16-07-15 om 11:29 schreef Daniel Vetter:
> On Thu, Jul 16, 2015 at 11:17:29AM +0200, Maarten Lankhorst wrote:
>> Op 16-07-15 om 11:19 schreef Daniel Vetter:
>>> On Thu, Jul 16, 2015 at 10:59:14AM +0200, Maarten Lankhorst wrote:
>>>> Cc: dri-devel@lists.freedesktop.org
>>>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>>>> ---
>>>>  drivers/gpu/drm/drm_atomic_helper.c | 7 +++++--
>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
>>>> index 0898afbc9e23..70e69904291d 100644
>>>> --- a/drivers/gpu/drm/drm_atomic_helper.c
>>>> +++ b/drivers/gpu/drm/drm_atomic_helper.c
>>>> @@ -667,8 +667,11 @@ drm_atomic_helper_update_legacy_modeset_state(struct drm_device *dev,
>>>>  	for_each_crtc_in_state(old_state, crtc, old_crtc_state, i) {
>>>>  		crtc->mode = crtc->state->mode;
>>>>  		crtc->enabled = crtc->state->enable;
>>>> -		crtc->x = crtc->primary->state->src_x >> 16;
>>>> -		crtc->y = crtc->primary->state->src_y >> 16;
>>>> +
>>>> +		if (drm_atomic_get_existing_plane_state(old_state, crtc->primary)) {
>>>> +			crtc->x = crtc->primary->state->src_x >> 16;
>>>> +			crtc->y = crtc->primary->state->src_y >> 16;
>>>> +		}
>>> What's the benefit here of only updating when something changed? The
>>> atomic state should be the master source so copying a few too many times
>>> shouldn't matter really.
>> Because you might not be holding plane lock, so primary->state may be garbage.
> Anyone who wants to touch primary plane must grab the crtc lock, so crtc
> lock would give you an implicit read lock. At least that's been my
> thinking, but it could be that the primary plane is used on some other
> crtc, and then this is indeed garbage.
This is only true if the plane is active. If there is none you can still update properties and
swap the plane state without locking the crtc.

> So maybe we need even more checks than what you propose:
>
>       if (drm_atomic_get_existing_plane_state(old_state, crtc->primary) &&
>           crtc->primary->state->crtc == crtc) {
>       	crtc->x = crtc->primary->state->src_x >> 16;
>       	crtc->y = crtc->primary->state->src_y >> 16;
>       }
>
> I think a comment explaining this would help (or at least in the commit
> message!).
But the primary and cursor planes are not allowed to move between crtc's?

~Maarten
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