[PATCH v2] drm/i915: fix screen flickering

[Matt Roper]

On Thu, May 14, 2015 at 09:16:39AM +0200, Thomas Gummerer wrote:
> 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.
> 
> Helped-by: Matt Roper <matthew.d.roper at intel.com>
> Signed-off-by: Thomas Gummerer <t.gummerer at gmail.com>

Seems like a reasonable short-term workaround and returns us to how the
code used to behaves.

Reviewed-by: Matt Roper <matthew.d.roper at intel.com>

> ---
> 
> > Sorry, I missed your patch when you first sent it.  That type of fix
> > looks like an okay workaround while we do a more in-depth rework of the
> > watermark system.  However I think your patch could cause a crash if we
> > disable the primary plane via the universal plane interface; if we do
> > that, p->pri.bytes_per_pixel is set to 0, but since we're now pretending
> > the primary plane is always enabled, ilk_wm_fbc() can eventually get
> > called and use that 0 in the denominator of a division operation.
> >
> > If you just squash the following change into your patch, I think it should be
> > safe:
> > [...]
> 
> Thank you very much for the suggestion, here is an updated version of the 
> patch.
> 
>  drivers/gpu/drm/i915/intel_pm.c | 24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> index fa4ccb3..555b896 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;
> -		p->pri.bytes_per_pixel = 0;
> -	}
> +	else
> +		p->pri.bytes_per_pixel = 4;
> +
> +	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;
>  
> -	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->pri.horiz_pixels = intel_crtc->config->pipe_src_w;
>  	p->cur.horiz_pixels = intel_crtc->base.cursor->state->crtc_w;
>  
> -- 
> 2.4.0.184.g8e1974e
> 

-- 
Matt Roper
Graphics Software Engineer
IoTG Platform Enabling & Development
Intel Corporation
(916) 356-2795