Re: [PATCH] drm/i915: Flush WC GGTT only on required platforms

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

On Fri, Oct 13, 2023 at 12:31:40PM +0200, Nirmoy Das wrote:
> gen8_ggtt_invalidate() is only needed for limitted set of platforms
> where GGTT is mapped as WC

I know there is supposed to be some kind hw snooping of the ggtt
pte writes to invalidate the tlb, but are we sure GFX_FLSH_CNTL
has no other side effects we depend on?

> otherwise this can cause unwanted
> side-effects on XE_HP platforms where GFX_FLSH_CNTL_GEN6 is not
> valid.
> 
> Fixes: d2eae8e98d59 ("drm/i915/dg2: Drop force_probe requirement")
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Jonathan Cavitt <jonathan.cavitt@intel.com>
> Cc: John Harrison <john.c.harrison@intel.com>
> Cc: Andi Shyti <andi.shyti@linux.intel.com>
> Cc: <stable@vger.kernel.org> # v6.2+
> Suggested-by: Matt Roper <matthew.d.roper@intel.com>
> Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_ggtt.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> index 4d7d88b92632..c2858d434bce 100644
> --- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
> +++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
> @@ -197,13 +197,17 @@ void gen6_ggtt_invalidate(struct i915_ggtt *ggtt)
>  
>  static void gen8_ggtt_invalidate(struct i915_ggtt *ggtt)
>  {
> +	struct drm_i915_private *i915 = ggtt->vm.i915;
>  	struct intel_uncore *uncore = ggtt->vm.gt->uncore;
>  
>  	/*
>  	 * Note that as an uncached mmio write, this will flush the
>  	 * WCB of the writes into the GGTT before it triggers the invalidate.
> +	 *
> +	 * Only perform this when GGTT is mapped as WC, see ggtt_probe_common().
>  	 */
> -	intel_uncore_write_fw(uncore, GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
> +	if (!IS_GEN9_LP(i915) && GRAPHICS_VER(i915) < 11)
> +		intel_uncore_write_fw(uncore, GFX_FLSH_CNTL_GEN6, GFX_FLSH_CNTL_EN);
>  }
>  
>  static void guc_ggtt_invalidate(struct i915_ggtt *ggtt)
> -- 
> 2.41.0

-- 
Ville Syrjälä
Intel