Re: [PATCH 1/2] drm/i915/fbc: fix CFB size calculation for gen8+

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

On Fri, Oct 21, 2016 at 01:55:45PM -0200, Paulo Zanoni wrote:
> Broadwell and newer actually compress up to 2560 lines instead of 2048
> (as documented in the FBC_CTL page). If we don't take this into
> consideration we end up reserving too little stolen memory for the
> CFB, so we may allocate something else (such as a ring) right after
> what we reserved, and the hardware will overwrite it with the contents
> of the CFB when FBC is active, causing GPU hangs. Another possibility
> is that the CFB may be allocated at the very end of the available
> space, so the CFB will overlap the reserved stolen area, leading to
> FIFO underruns.
> 
> This bug has always been a problem on BDW (the only affected platform
> where FBC is enabled by default), but it's much easier to reproduce
> since the following commit:
>     commit c58b735fc762e891481e92af7124b85cb0a51fce
>     Author: Chris Wilson <chris@chris-wilson.co.uk>
>     Date:   Thu Aug 18 17:16:57 2016 +0100
>         drm/i915: Allocate rings from stolen
> 
> Of course, you can only reproduce the bug if your screen is taller
> than 2048 lines.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98213
> Fixes: a98ee79317b4 ("drm/i915/fbc: enable FBC by default on HSW and BDW")
> Cc: <stable@vger.kernel.org> # v4.6+
> Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/intel_fbc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
> index 617189a..6345cb8 100644
> --- a/drivers/gpu/drm/i915/intel_fbc.c
> +++ b/drivers/gpu/drm/i915/intel_fbc.c
> @@ -104,8 +104,10 @@ static int intel_fbc_calculate_cfb_size(struct drm_i915_private *dev_priv,
>  	int lines;
>  
>  	intel_fbc_get_plane_source_size(cache, NULL, &lines);
> -	if (INTEL_INFO(dev_priv)->gen >= 7)
> +	if (INTEL_GEN(dev_priv) == 7)
>  		lines = min(lines, 2048);
> +	else if (INTEL_GEN(dev_priv) >= 8)
> +		lines = min(lines, 2560);
>  
>  	/* Hardware needs the full buffer stride, not just the active area. */
>  	return lines * cache->fb.stride;
> -- 
> 2.7.4
> 
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-- 
Ville Syrjälä
Intel OTC
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