Re: [PATCH] drm/i915: workaround bad DSL readout v3

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 22 Sep 2015, Jesse Barnes <jbarnes@virtuousgeek.org> 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)
> v3: use break instead of goto (Ville)
>     update comment with workaround details (Ville)
>
> References: https://bugs.freedesktop.org/show_bug.cgi?id=91579
> Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

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

Sorry for the delay, I failed to notice the patch had been reviewed. It
would have helped if the patch had the Reviewed-by: tag already in
place, or the reviewer (or someone else pointing at the review) had
replied with the Reviewed-by: tag. I spend very little time looking at
individual patches, especially if the appearance is that there's
discussion going on.

BR,
Jani.


> ---
>  drivers/gpu/drm/i915/i915_irq.c | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> index 1f53061..a8aa797 100644
> --- a/drivers/gpu/drm/i915/i915_irq.c
> +++ b/drivers/gpu/drm/i915/i915_irq.c
> @@ -697,6 +697,32 @@ 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 just before 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.
> +	 *
> +	 * The nature of this problem means we can't simply check the ISR
> +	 * bit and return the vblank start value; nor can we use the scanline
> +	 * debug register in the transcoder as it appears to have the same
> +	 * problem.  We may need to extend this to include other platforms,
> +	 * but so far testing only shows the problem on HSW.
> +	 */
> +	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;
> +				break;
> +			}
> +		}
> +	}
> +
> +	/*
>  	 * See update_scanline_offset() for the details on the
>  	 * scanline_offset adjustment.
>  	 */
> -- 
> 1.9.1
>
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-- 
Jani Nikula, Intel Open Source Technology Center
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