Re: [PATCH] drm/i915: Fix IPS related flicker

[Kenneth Graunke <kenneth@whitecape.org>]

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On Thursday, May 21, 2015 12:53:03 PM Rodrigo Vivi wrote:
> We cannot let IPS enabled with no plane on the pipe:
> 
> BSpec: "IPS cannot be enabled until after at least one plane has
> been enabled for at least one vertical blank." and "IPS must be
> disabled while there is still at least one plane enabled on the
> same pipe as IPS.
> 
> However this shortcut path to make primary plane invisible when
> updating primary plane was leting IPS enabled while there was no
> other plane enabled on the pipe causing flickerings that we were
> believing that it was caused by that other restriction where
> ips cannot be used when pixel rate is greater than 95% of cdclok.
> 
> Reference: https://bugs.freedesktop.org/show_bug.cgi?id=85583
> Cc: Joe Konno <joe.konno@intel.com>
> Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_display.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 9d2d6fb..5519d56 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -2789,6 +2789,19 @@ static void ironlake_update_primary_plane(struct drm_crtc *crtc,
>  	int pixel_size;
>  
>  	if (!visible || !fb) {
> +		/*
> +		 * This shortcut path disables the primary plane making
> +		 * IPS really sad and lost when it is enabled alone with no
> +		 * plane around on the pipe. So, for now it is safe to disable
> +		 * IPS here.
> +		 /
> +		 /*
> +		 * FIXME: Any other plane enabled should be enough so it would
> +		 * be better to check if there is really no sprite or
> +		 * cursor around.
> +		 */
> +		hsw_disable_ips(intel_crtc);
> +
>  		I915_WRITE(reg, 0);
>  		I915_WRITE(DSPSURF(plane), 0);
>  		POSTING_READ(reg);

This fixes a bug on my Lenovo X250 (Broadwell GT2).

1. Boot
2. startx using SNA and KDE 5
3. Quit X

Prior to this patch, the screen was all scrambled.  With this patch, it
works perfectly.

Thanks, Rodrigo!

Tested-by: Kenneth Graunke <kenneth@whitecape.org>

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