Re: [PATCH v2] drm/i915: fix screen flickering

[Jani Nikula]

On Thu, 14 May 2015, Matt Roper <matthew.d.roper@xxxxxxxxx> wrote:
> 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@xxxxxxxxx>
>> Signed-off-by: Thomas Gummerer <t.gummerer@xxxxxxxxx>
>
> Seems like a reasonable short-term workaround and returns us to how the
> code used to behaves.
>
> Reviewed-by: Matt Roper <matthew.d.roper@xxxxxxxxx>

Pushed to drm-intel-fixes, thanks for the patch and review.

BR,
Jani.


>
>> ---
>> 
>> > 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

-- 
Jani Nikula, Intel Open Source Technology Center
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