Re: [Intel-gfx] [PATCH 1/2] drm/i915/sseu: Don't overallocate subslice storage

[Lucas De Marchi <lucas.demarchi@intel.com>]

On Fri, Mar 11, 2022 at 12:43:40PM -0800, Matt Roper wrote:
>On Fri, Mar 11, 2022 at 12:38:17PM -0800, Matt Roper wrote:
>> On Fri, Mar 11, 2022 at 11:00:09AM -0800, Lucas De Marchi wrote:
>> > On Thu, Mar 10, 2022 at 10:15:42PM -0800, Matt Roper wrote:
>> > > Xe_HP removed "slice" as a first-class unit in the hardware design.
>> > > Instead we now have a single pool of subslices (which are now referred
>> > > to as "DSS") that different hardware units have different ways of
>> > > grouping ("compute slices," "geometry slices," etc.).  For the purposes
>> > > of topology representation, we treat Xe_HP-based platforms as having a
>> > > single slice that contains all of the platform's DSS.  There's no need
>> > > to allocate storage space for (max legacy slices * max dss); let's
>> > > update some of our macros to minimize the storage requirement for sseu
>> > > topology.  We'll also document some of the constants to make it a little
>> > > bit more clear what they represent.
>> > >
>> > > Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
>> > > ---
>> > > drivers/gpu/drm/i915/gt/intel_engine_types.h |  2 +-
>> > > drivers/gpu/drm/i915/gt/intel_sseu.h         | 47 +++++++++++++++-----
>> > > 2 files changed, 36 insertions(+), 13 deletions(-)
>> > >
>> > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
>> > > index 4fbf45a74ec0..f9e246004bc0 100644
>> > > --- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
>> > > +++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
>> > > @@ -645,7 +645,7 @@ intel_engine_has_relative_mmio(const struct intel_engine_cs * const engine)
>> > >
>> > > #define for_each_instdone_gslice_dss_xehp(dev_priv_, sseu_, iter_, gslice_, dss_) \
>> > > 	for ((iter_) = 0, (gslice_) = 0, (dss_) = 0; \
>> > > -	     (iter_) < GEN_MAX_SUBSLICES; \
>> > > +	     (iter_) < GEN_SS_MASK_SIZE; \
>> > > 	     (iter_)++, (gslice_) = (iter_) / GEN_DSS_PER_GSLICE, \
>> > > 	     (dss_) = (iter_) % GEN_DSS_PER_GSLICE) \
>> > > 		for_each_if(intel_sseu_has_subslice((sseu_), 0, (iter_)))
>> > > diff --git a/drivers/gpu/drm/i915/gt/intel_sseu.h b/drivers/gpu/drm/i915/gt/intel_sseu.h
>> > > index 8a79cd8eaab4..4f59eadbb61a 100644
>> > > --- a/drivers/gpu/drm/i915/gt/intel_sseu.h
>> > > +++ b/drivers/gpu/drm/i915/gt/intel_sseu.h
>> > > @@ -15,26 +15,49 @@ struct drm_i915_private;
>> > > struct intel_gt;
>> > > struct drm_printer;
>> > >
>> > > -#define GEN_MAX_SLICES		(3) /* SKL upper bound */
>> > > -#define GEN_MAX_SUBSLICES	(32) /* XEHPSDV upper bound */
>> > > -#define GEN_SSEU_STRIDE(max_entries) DIV_ROUND_UP(max_entries, BITS_PER_BYTE)
>> > > -#define GEN_MAX_SUBSLICE_STRIDE GEN_SSEU_STRIDE(GEN_MAX_SUBSLICES)
>> > > -#define GEN_MAX_EUS		(16) /* TGL upper bound */
>> > > -#define GEN_MAX_EU_STRIDE GEN_SSEU_STRIDE(GEN_MAX_EUS)
>> > > +/*
>> > > + * Maximum number of legacy slices.  Legacy slices no longer exist starting on
>> > > + * Xe_HP ("gslices," "cslices," etc. on Xe_HP and beyond are a different
>> > > + * concept and are not expressed through fusing).
>> > > + */
>> > > +#define GEN_MAX_LEGACY_SLICES		3
>> > > +
>> > > +/*
>> > > + * Maximum number of subslices that can exist within a legacy slice.  This is
>> > > + * only relevant to pre-Xe_HP platforms (Xe_HP and beyond use the GEN_MAX_DSS
>> > > + * value below).
>> > > + */
>> > > +#define GEN_MAX_LEGACY_SUBSLICES	6
>> >
>> > instead of calling the old legacy, maybe just add the XEHP_ prefix to
>> > the new ones?
>>
>> Maybe a "HSW_" prefix on the old ones would be better?  People still use
>> the termm 'subslice' in casual discussion when talking about DSS, so I
>> want to somehow distinguish that what we're talking about here is a
>> different, older design than we have on modern platforms.
>
>Hmm, or maybe just "GEN_MAX_SUBSLICES_PER_LEGACY_SLICE" to tie it into
>the slice definition above?

I'm not too fond of calling it "legacy" when everywhere else in the driver
we just use the platform as prefix/suffix. Some may see legacy as
< ver 12, others as 12.50, etc.

Well... but I will leave that up to you if you are convinced one is
better than the other.

thanks
Lucas De Marchi