[Intel-gfx] [PATCH] drm/i915: Flush WC GGTT only on required platforms

[Nirmoy Das]

Hi Ville,

On 10/13/2023 12:50 PM, Ville Syrjälä wrote:
> 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?

I spent some time searching through the gfxspec. This GFX_FLSH_CNTL 
register only seems to be for

invalidating TLB for GUnit  and (from git log ) we started to do that to 
enable WC based GGTT updates.


So if I am not missing anything obvious then this should be safe.


Regards,

Nirmoy

>
>> 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 at intel.com>
>> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
>> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
>> Cc: Jani Nikula <jani.nikula at linux.intel.com>
>> Cc: Jonathan Cavitt <jonathan.cavitt at intel.com>
>> Cc: John Harrison <john.c.harrison at intel.com>
>> Cc: Andi Shyti <andi.shyti at linux.intel.com>
>> Cc: <stable at vger.kernel.org> # v6.2+
>> Suggested-by: Matt Roper <matthew.d.roper at intel.com>
>> Signed-off-by: Nirmoy Das <nirmoy.das at 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