[PATCH/RFC] i915: fix screen flickering

[Thomas Gummerer]

Thomas Gummerer <t.gummerer at gmail.com> writes:

> Commit c9f038a1a592 ("drm/i915: Don't assume primary & cursor are
> always on for wm calculation (v4)") fixes a null pointer dereference.
> Setting the primary and cursor panes to false in
> ilk_compute_wm_parameters to false does however give the following
> errors in the kernel log and causes the screen to flicker.
>
> [  101.133716] [drm:intel_set_cpu_fifo_underrun_reporting [i915]]
> *ERROR* uncleared fifo underrun on pipe A
> [  101.133725] [drm:intel_cpu_fifo_underrun_irq_handler [i915]]
> *ERROR* CPU pipe A FIFO underrun
>
> Always setting the panes to enabled fixes this error.
>
> Signed-off-by: Thomas Gummerer <t.gummerer at gmail.com>
> ---
> Hi,
>
> I've noticed the regression in v4.1-rc1.  I'm not sure this is the best
> solution, but it's the best I could come up with in the time I had
> available.

I forgot to mention, the bug only occurs after suspending the system and
waking it up again.  In that case, moving the mouse pointer around on
the build in laptop screen triggers the flickering, while the external
monitor works just fine.

Might be unrelated, but I only managed to trigger the bug when the
system is suspended after the synergy server has started.

>  drivers/gpu/drm/i915/intel_pm.c | 22 ++++++++++------------
>  1 file changed, 10 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index fa4ccb3..377df60 100644
> --- a/drivers/gpu/drm/i915/intel_pm.c
> +++ b/drivers/gpu/drm/i915/intel_pm.c
> @@ -2045,22 +2045,20 @@ static void ilk_compute_wm_parameters(struct drm_crtc *crtc,
>  	p->pipe_htotal = intel_crtc->config->base.adjusted_mode.crtc_htotal;
>  	p->pixel_rate = ilk_pipe_pixel_rate(dev, crtc);
>  
> -	if (crtc->primary->state->fb) {
> -		p->pri.enabled = true;
> +	if (crtc->primary->state->fb)
>  		p->pri.bytes_per_pixel =
>  			crtc->primary->state->fb->bits_per_pixel / 8;
> -	} else {
> -		p->pri.enabled = false;
> +	else
>  		p->pri.bytes_per_pixel = 0;
> -	}
>  
> -	if (crtc->cursor->state->fb) {
> -		p->cur.enabled = true;
> -		p->cur.bytes_per_pixel = 4;
> -	} else {
> -		p->cur.enabled = false;
> -		p->cur.bytes_per_pixel = 0;
> -	}
> +	p->cur.bytes_per_pixel = 4;
> +	/*
> +	 * TODO: for now, assume primary and cursor planes are always enabled.
> +	 * Setting them to false makes the screen flicker.
> +	 */
> +	p->pri.enabled = true;
> +	p->cur.enabled = true;
> +
>  	p->pri.horiz_pixels = intel_crtc->config->pipe_src_w;
>  	p->cur.horiz_pixels = intel_crtc->base.cursor->state->crtc_w;
>  
> -- 
> 2.1.0.264.g0463184.dirty