[Intel-gfx] [PATCH] drm/i915: Workaround CHV pipe C cursor fail

[Daniel Vetter]

On Fri, Dec 18, 2015 at 07:24:39PM +0200, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Turns out CHV pipe C was glued on somewhat poorly, and there's something
> wrong with the cursor. If the cursor straddles the left screen edge,
> and is then moved away from the edge or disabled, the pipe will often
> underrun. If enough underruns are triggered quickly enough the pipe
> will fall over and die (it just scans out a solid color and reports
> a constant underrun). We need to turn the disp2d power well off and
> on again to recover the pipe.
> 
> None of that is very nice for the user, so let's just refuse to place
> the cursor in the compromised position. The ddx appears to fall back
> to swcursor when the ioctl returns an error, so theoretically there's
> no loss of functionality for the user (discounting swcursor bugs).
> I suppose most cursors images actually have the hotspot not exactly
> at 0,0 so under typical conditions the fallback will in fact kick in
> as soon as the cursor touches the left edge of the screen.
> 
> Any atomic compositor should anyway be prepared to fall back to
> GPU composition when things don't work out, so there should be no
> problem with those.
> 
> Other things that I tried to solve this include flipping all
> display related clock gating knobs I could find, increasing the
> minimum gtt alignment all the way up to 512k. I also tried to see
> if there are more specific screen coordinates that hit the bug, but
> the findings were somewhat inconclusive. Sometimes the failures
> happen almost across the whole left edge, sometimes more at the very
> top and around the bottom half. I wasn't able to find any real pattern
> to these variations, so it seems our only choice is to just refuse
> to straddle the left screen edge at all.
> 
> Cc: stable at vger.kernel.org
> Cc: Jason Plum <max at warheads.net>
> Testcase: igt/kms_chv_cursor_fail
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92826
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Well can't really review with Bspec and I don't expect we can get
confirmation from hw engineers. But does what it says in the commit
message.

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  drivers/gpu/drm/i915/intel_display.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index abd2d2944022..8acc66b95139 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -14059,6 +14059,7 @@ intel_check_cursor_plane(struct drm_plane *plane,
>  	struct drm_crtc *crtc = crtc_state->base.crtc;
>  	struct drm_framebuffer *fb = state->base.fb;
>  	struct drm_i915_gem_object *obj = intel_fb_obj(fb);
> +	enum pipe pipe = to_intel_plane(plane)->pipe;
>  	unsigned stride;
>  	int ret;
>  
> @@ -14092,6 +14093,22 @@ intel_check_cursor_plane(struct drm_plane *plane,
>  		return -EINVAL;
>  	}
>  
> +	/*
> +	 * There's something wrong with the cursor on CHV pipe C.
> +	 * If it straddles the left edge of the screen then
> +	 * moving it away from the edge or disabling it often
> +	 * results in a pipe underrun, and often that can lead to
> +	 * dead pipe (constant underrun reported, and it scans
> +	 * out just a solid color). To recover from that, the
> +	 * display power well must be turned off and on again.
> +	 * Refuse the put the cursor into that compromised position.
> +	 */
> +	if (IS_CHERRYVIEW(plane->dev) && pipe == PIPE_C &&
> +	    state->visible && state->base.crtc_x < 0) {
> +		DRM_DEBUG_KMS("CHV cursor C not allowed to straddle the left screen edge\n");
> +		return -EINVAL;
> +	}
> +
>  	return 0;
>  }
>  
> -- 
> 2.4.10
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch