[Intel-xe] [PATCH v6 1/5] drm/xe/pat: trim the xelp PAT table

[Lucas De Marchi]

On Thu, Sep 28, 2023 at 09:38:39AM -0700, Matt Roper wrote:
>On Thu, Sep 28, 2023 at 08:04:18AM -0500, Lucas De Marchi wrote:
>> On Thu, Sep 28, 2023 at 11:05:39AM +0100, Matthew Auld wrote:
>> > We don't seem to use the 4-7 pat indexes, even though they are defined
>> > by the HW. In the next patch userspace will be able to directly set the
>> > pat_index as part of vm_bind and we don't want to allow setting 4-7.
>> > Simplest is to just ignore them here.
>>
>> But why do we want to leave them in un-programmed state? Wouldn't it be
>> more future-proof we we program them to something like UC?
>
>By reducing the table here, we prevent these indices from being used by
>the uapi, so it's impossible for userspace to ask for them.  PAT only
>gets used through the uapi and can't be specified inside instructions
>like MOCS can.
>
>Also, even if we did think there was some way for userspace to get
>around us and accidentally utilize these, the hardware defaults are
>already set the way we want (maximal caching).  Maximal caching is also
>the strategy we use for uninitialized MOCS entries because the
>expectation is that userspace is a bit less likely to see a regression
>if unused entries become used in the future and switch from more cached
>to less cached than they are if they change in the other direction.

ok, makes sense. Thanks for the explanation.

Reviewed-by: Lucas De Marchi <lucas.demarchi at intel.com>


Lucas De Marchi

>
>
>Matt
>
>>
>> Lucas De Marchi
>>
>> >
>> > Suggested-by: Matt Roper <matthew.d.roper at intel.com>
>> > Signed-off-by: Matthew Auld <matthew.auld at intel.com>
>> > Cc: Pallavi Mishra <pallavi.mishra at intel.com>
>> > Cc: Lucas De Marchi <lucas.demarchi at intel.com>
>> > Reviewed-by: Matt Roper <matthew.d.roper at intel.com>
>> > ---
>> > drivers/gpu/drm/xe/xe_pat.c | 4 ----
>> > 1 file changed, 4 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/xe/xe_pat.c b/drivers/gpu/drm/xe/xe_pat.c
>> > index 869aee64641f..36b79d675e54 100644
>> > --- a/drivers/gpu/drm/xe/xe_pat.c
>> > +++ b/drivers/gpu/drm/xe/xe_pat.c
>> > @@ -42,10 +42,6 @@ static const u32 xelp_pat_table[] = {
>> > 	[1] = XELP_PAT_WC,
>> > 	[2] = XELP_PAT_WT,
>> > 	[3] = XELP_PAT_UC,
>> > -	[4] = XELP_PAT_WB,
>> > -	[5] = XELP_PAT_WB,
>> > -	[6] = XELP_PAT_WB,
>> > -	[7] = XELP_PAT_WB,
>> > };
>> >
>> > static const u32 xehpc_pat_table[] = {
>> > --
>> > 2.41.0
>> >
>
>-- 
>Matt Roper
>Graphics Software Engineer
>Linux GPU Platform Enablement
>Intel Corporation