Re: [PATCH] drm/i915: workaround bad DSL readout v2

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Sep 10, 2015 at 02:38:53PM -0700, Jesse Barnes wrote:
> On HSW at least (still testing other platforms, but should be harmless
> elsewhere), the DSL reg reads back as 0 when read around vblank start
> time.  This ends up confusing the atomic start/end checking code, since
> it causes the update to appear as if it crossed a frame count boundary.
> Avoid the problem by making sure we don't return scanline_offset from
> the get_crtc_scanline function.  In moving the code there, I add to add
> an additional delay since it could be called and have a legitimate 0
> result for some time (depending on the pixel clock).
> 
> v2: move hsw dsl read hack to get_crtc_scanline (Ville)
> 
> References: https://bugs.freedesktop.org/show_bug.cgi?id=91579
> Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
> ---
>  drivers/gpu/drm/i915/i915_irq.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> index 90bc6c2..97e5d52 100644
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -697,6 +697,27 @@ static int __intel_get_crtc_scanline(struct intel_crtc *crtc)
>  		position = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) & DSL_LINEMASK_GEN3;
>  
>  	/*
> +	 * On HSW, the DSL reg (0x70000) appears to return 0 if we
> +	 * read it right around the start of vblank.  So try it again
> +	 * so we don't accidentally end up spanning a vblank frame
> +	 * increment, causing the pipe_update_end() code to squak at us.
> +	 */
> +	if (IS_HASWELL(dev) && !position) {
> +		int i, temp;
> +
> +		for (i = 0; i < 100; i++) {
> +			udelay(1);
> +			temp = __raw_i915_read32(dev_priv, PIPEDSL(pipe)) &
> +				DSL_LINEMASK_GEN3;
> +			if (temp != position) {
> +				position = temp;
> +				goto out;
> +			}
> +		}
> +	}

Hmm. Another idea. If it always happens at start of vbl, maybe have a
look at the ISR. If scanline reads 0, but ISR says we're in vblank, just
return vblank_start. That's assming ISR would in fact show that it's in
vblank when this happens.

> +
> +out:
> +	/*
>  	 * See update_scanline_offset() for the details on the
>  	 * scanline_offset adjustment.
>  	 */
> -- 
> 1.9.1
> 
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-- 
Ville Syrjälä
Intel OTC
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