Re: [PATCH] drm/i915/gt: Program mocs:63 for cache eviction on gen9

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

On Thu, Nov 26, 2020 at 02:08:41PM +0000, Chris Wilson wrote:
> Ville noticed that the last mocs entry is used unconditionally by the HW
> when it performs cache evictions, and noted that while the value is not
> meant to be writable by the driver, we should program it to a reasonable
> value nevertheless.
> 
> As it turns out, we can change the value of mocs:63 and the value we
> were programming into it would cause hard hangs in conjunction with
> atomic operations.
> 
> v2: Add details from bspec about how it is used by HW
> 
> Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2707
> Fixes: 3bbaba0ceaa2 ("drm/i915: Added Programming of the MOCS")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Jason Ekstrand <jason@jlekstrand.net>
> Cc: <stable@vger.kernel.org> # v4.3+
> ---
>  drivers/gpu/drm/i915/gt/intel_mocs.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_mocs.c b/drivers/gpu/drm/i915/gt/intel_mocs.c
> index 254873e1646e..26cedde80476 100644
> --- a/drivers/gpu/drm/i915/gt/intel_mocs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_mocs.c
> @@ -131,7 +131,19 @@ static const struct drm_i915_mocs_entry skl_mocs_table[] = {
>  	GEN9_MOCS_ENTRIES,
>  	MOCS_ENTRY(I915_MOCS_CACHED,
>  		   LE_3_WB | LE_TC_2_LLC_ELLC | LE_LRUM(3),
> -		   L3_3_WB)
> +		   L3_3_WB),
> +
> +	/*
> +	 * mocs:63
> +	 * - used by the L3 for all its evictions.
> +	 *   Thus it is expected to allow LLC cacheability to enable coherent
> +	 *   flows to be maintained.
> +	 * - used to force L3 uncachable cycles.
> +	 *   Thus it is expected to make the surce L3 uncacheable.

"surce"?

> +	 */
> +	MOCS_ENTRY(63,
> +		   LE_3_WB | LE_TC_1_LLC | LE_LRUM(3),
> +		   L3_1_UC)
>  };
>  
>  /* NOTE: the LE_TGT_CACHE is not used on Broxton */
> -- 
> 2.20.1

-- 
Ville Syrjälä
Intel