[Intel-gfx] [PATCH] drm/i915: Fix screen flickering on X

[Ander Conselvan De Oliveira]

On Sat, 2015-04-11 at 19:17 -0300, Ismael Luceno wrote:
> A bisect showed that commit 32b7eeec4d1e861230b09d437e95d76c86ff4a68
> introduced the issue.
> 
> The issue starts as soon as X takes control of the screen, even if just
> a plain X doing nothing, so based on the code touched by the commit I
> thought it had to be related to the so called "hardware cursor". I
> confirmed it when hiding the cursor made the flickering go away.
> 
> The aforementioned commit removed some suspicious code, and the
> Programmer's Reference Manual confirmed my suspicion:
> 
> "Incorrectly programmed watermark values can result in screen corruption.
> 
> The watermarks should be calculated and programmed when any of the
> watermark calculation inputs change. This includes planes enabling or
> disabling, plane source format or size changing, etc."
> 
> So I'm re-adding the few lines that update the watermarks after a cursor
> size change.
> 
> Signed-off-by: Ismael Luceno <ismael at iodev.co.uk>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index f75173c..e23f062 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -12258,6 +12258,7 @@ intel_commit_cursor_plane(struct drm_plane *plane,
>  	struct intel_crtc *intel_crtc;
>  	struct intel_plane *intel_plane = to_intel_plane(plane);
>  	struct drm_i915_gem_object *obj = intel_fb_obj(state->base.fb);
> +	unsigned old_width;
>  	uint32_t addr;
>  
>  	crtc = crtc ? crtc : plane->crtc;
> @@ -12282,11 +12283,15 @@ intel_commit_cursor_plane(struct drm_plane *plane,
>  	intel_crtc->cursor_addr = addr;
>  	intel_crtc->cursor_bo = obj;
>  update:
> +	old_width = intel_crtc->cursor_width;
>  	intel_crtc->cursor_width = state->base.crtc_w;
>  	intel_crtc->cursor_height = state->base.crtc_h;
>  
> -	if (intel_crtc->active)
> +	if (intel_crtc->active) {
> +		if (old_width != intel_crtc->cursor_width)
> +			intel_update_watermarks(crtc);
>  		intel_crtc_update_cursor(crtc, state->visible);
> +	}


We can't call intel_update_watermarks() from here because we commit
under vblank evasion (with interrupts disabled) and some parts of that
function might sleep. That's why that call to it was moved to
intel_begin_crtc_commit(). The check for change in width was moved to
intel_check_cursor_plane() (where the intel_crtc->atomic.update_wm flag
is set).

So the question here is why that logic is not working. Could you check
if update_wm is being set and intel_update_watermarks() called from
intel_begin_crtc_commit()?

Thanks,
Ander