[Mesa-dev] [Intel-gfx] [PATCH] drm/i915/gt: Add separate MOCS table for Gen12 devices other than TGL/RKL

[Ville Syrjälä]

On Thu, Sep 09, 2021 at 10:15:56AM -0700, Matt Roper wrote:
> On Thu, Sep 09, 2021 at 06:09:55PM +0300, Ville Syrjälä wrote:
> > On Thu, Sep 09, 2021 at 08:00:02AM -0700, Matt Roper wrote:
> > > On Thu, Sep 09, 2021 at 05:39:26PM +0300, Ville Syrjälä wrote:
> > > > On Thu, Sep 09, 2021 at 07:29:33AM -0700, Matt Roper wrote:
> > > > > On Thu, Sep 09, 2021 at 04:58:50PM +0300, Ville Syrjälä wrote:
> > > > > > On Tue, Sep 07, 2021 at 11:19:29AM -0700, Matt Roper wrote:
> > > > > > > On Tue, Sep 07, 2021 at 08:41:06PM +0300, Ville Syrjälä wrote:
> > > > > > > > On Tue, Sep 07, 2021 at 10:27:28AM -0700, Matt Roper wrote:
> > > > > > > > > On Tue, Sep 07, 2021 at 10:46:39PM +0530, Ayaz A Siddiqui wrote:
> > > > > > > > > > MOCS table of TGL/RKL has MOCS[1] set to L3_UC.
> > > > > > > > > > While for other gen12 devices we need to set MOCS[1] as L3_WB,
> > > > > > > > > > So adding a new MOCS table for other gen 12 devices eg. ADL.
> > > > > > > > > > 
> > > > > > > > > > Fixes: cfbe5291a189 ("drm/i915/gt: Initialize unused MOCS entries with device specific values")
> > > > > > > > > > Cc: Matt Roper <matthew.d.roper at intel.com>
> > > > > > > > > > Signed-off-by: Ayaz A Siddiqui <ayaz.siddiqui at intel.com>
> > > > > > > > > 
> > > > > > > > > Yep, we overlooked that the TGL table still had an explicit entry for
> > > > > > > > > I915_MOCS_PTE and wasn't just using an implicit 'unused_entries' lookup
> > > > > > > > > for MOCS[1].  The new table is the same as the TGL table, just with
> > > > > > > > > I915_MOCS_PTE (1) removed.
> > > > > > > > 
> > > > > > > > And just how are people planning on handling display cacheability
> > > > > > > > control without a PTE MOCS entry? Is Mesa/etc. already making all
> > > > > > > > external bos uncached on these platforms just in case we might
> > > > > > > > scan out said bo?
> > > > > > > 
> > > > > > > MOCS entry 1 has never been considered a valid MOCS table entry on gen12
> > > > > > > platforms (despite the old #define, it's not actually related to PTE,
> > > > > > > display, etc. anymore).
> > > > > > 
> > > > > > So can someone finally explain to me how we're supposed to cache
> > > > > > anything that might become a scanout buffer later (eg. window system
> > > > > > buffers)? Or are we just making everything like that UC now, and is
> > > > > > everyone happy with that? Is userspace actually following that?
> > > > > 
> > > > > Table entry #1 has never had anything to do with scanout on gen12+.  I
> > > > > would assume that UMDs are either using the display entry in the MOCS
> > > > > table (which is 61 on gen12+) or some other UC entry.
> > > > 
> > > > If 61 is meant to to be the new PTE entry wy hasn't it been defines as
> > > > such in the code? And I know for a fact that userspace (Mesa) is not
> > > 
> > > There is no "PTE" entry anymore.  But 61 is already documented as
> > > "displayable" in both the spec and the code:
> > > 
> > >         /* HW Special Case (Displayable) */                                      
> > >         MOCS_ENTRY(61,                             
> > 
> > Why is it called a "HW special case"? I don't think there's any hw
> > magic in there?
> > 
> > And why aren't we setting it to PTE to get some cacheability for
> > window back buffers and such?
> 
> Who is "we" here?

We who care about the performance of the system.

> The MOCS table is a pre-defined set of per-platform
> magic numbers.  The software teams don't get to decide what the values
> are, we just program the hardware with the per-platform numbers that
> have been agreed upon as part of a platform-wide stack (everything from
> low-level firmware to high level userspace should be working from the
> same table, defined in the bspec).

The magic numbers must be based on something. If that something is
purely Windows behaviour/performance then we might be shooting
ourselves in the foot here.

> 
> Once we know what the per-platform magic numbers are, we're supposed to
> pick the table entry that matches the behavior we're trying to
> accomplish.  If you want some specific level of cacheability, then you
> select a table row that gives you that.  Maybe 61 isn't the best
> setting, I don't know; userspace can pick whichever defined setting is
> actually best, using the data from the table.  But table row #1 is
> already well-documented as reserved/dontuse across the full stack; the
> fact that row #1 had values similar to PTE on Icelake hardware doesn't
> carry forward to any post-gen11 platform.

The only way you can get LLC cacheability for an external BO (window
back buffers and such) is by using a MOCS entry that directs the hardware
to consult the PTEs. Otherwise the client doing the rendering would have
to know ahead of time whether the buffer is going to be directly scanned
out by the compositor or not, for which there is no protocol in
X or wayland.

Historically I believe LLC cacheability has been on average a win.
Some workloads can do better with UC though. So if we are giving up
on LLC cacheability we should have some numbers to back up that
decision so that we're not dropping tons of performance on the floor.

> > > > using entry 61. I think there is a massive communication gap here
> > > > where everyone just seems to assume the other side is doing something.
> > > > 
> > > > Could someone actually come up with a clear abi definition for this
> > > > and get all the stakeholders to sign off on it?
> > > 
> > > The agreement between the i915 team, various userspace teams, Windows
> > > driver team, hardware architects, software architects, and bspec writers
> > > was just completed; that's what triggered the kernel updates here (and
> > > I'm guessing is triggering similar work on the UMD side).  It's also why
> > > we held off on removing the force_probe flag on ADL until now since we
> > > couldn't consider enablement of the platform complete until the
> > > agreement and definitions here was finalized.
> > 
> > Can we get that agreement visible on the mailing list? Since MOCS is
> > abi I don't see why we shouldn't follow the normal abi rules for these,
> > ie. post to dri-devel, get acks from relevant people, links to agreed
> > userspace changes, etc.
> 
> The ABI design here was designed and agreed upon years ago, during early
> gen11 development.  The ABI design is that the kernel driver will
> faithfully initialize the hardware with the pre-determined set of magic
> numbers documented by the hardware team.  Since these are
> well-documented and unchanging numbers per-platform, there's no
> ambiguity for userspace, firmware, etc. about what a specific mocs index
> means, and no need to provide additional ABI for userspace to query what
> the kernel used in each row or anything like that.  The specific magic
> numbers are also ABI in the sense that we can't change the set of
> defined values once they're set for a platform (and it's been a long
> road to get the hardware and other OS software teams to understand and
> agree to this requirement), but we don't get to define or overrule what
> the initial values and order of those magic numbers are.

And this apporach has clearly not worked considering userspace and
kernel have not agreed on what the abi is. We need to do better.

> 
> What is a bit vague in the formal documentation is what should be done
> about the reserved/dontuse table entries.  In theory it wouldn't matter
> since they'd never be used anyway, but in reality userspace can still
> use them by accident, such as by forgetting to update their MOCS
> selection logic from past platforms (e.g., still trying to use row #1 on
> platforms where it isn't defined).  Given that it's legal for entries to
> be added to MOCS tables, but never removed/modified, it follows that we
> should always initialize the undefined entries to fully cached; if a
> MOCS table update happens in the future and new rows show up, they can
> only become more coherent, and any userspace software that was
> incorrectly trying to use them previously will remain functionally
> correct.
> 
> What you're proposing would be a change to existing ABI --- instead of
> following the agreed upon contract, i915 would start defining its own
> set of magic numbers that potentially contradict the documentation that
> every other team is depending on.  We already made this mistake on
> TGL/RKL, so due to an i915 bug we're outside the spec; if entry #1 ever
> becomes a formally defined setting in the future, the rest of the
> software stack will need to explicitly work around i915's bug since we
> can't fix it now without breaking ABI.
> 
> If you really want to redefine how the MOCS ABI works and have i915 no
> longer follow the current contract, I think you need to do the
> following:

I want the abi to be actually defined properly, and some assurance that
all the stakeholders implement it correctly. Following the proper abi
rules for kernel development would guarantee that.

I would also like if the abi can give us the best performance rather
than potentially crippling it. Ie. I would expect to have a PTE MOCS
setting for potential scanout buffers, or some proof that pure UC
will in fact be a better choice.

> 
>  * Re-add force_probe requirement to ADL and add appropriate Fixes: so
>    that the platform isn't enabled yet.
>  * Document clearly how you want i915 to select the MOCS settings it
>    uses for each table row if we're no longer going to follow the
>    documented values in the bspec.
>  * Provide a way for userspace to determine how i915 has defined the
>    MOCS settings (since they can no longer rely on us following the
>    previously agreed upon contract).
>  * Get acks from all the userspace teams on the new direction you're
>    proposing.
>  * Get an ack from the GuC team to make sure that programming
>    MOCS values differently than documented in the bspec won't have any
>    kind of impact on their operation.
> 
> Alternatively, you could lobby for a new table row #1 to be added to the
> formal MOCS table for gen12. It's legal for new MOCS entries to show up
> in the future, so if that happens, we can follow up with the
> corresponding change in i915; since we'll be moving to a more coherent
> value (from today's fully cached entry), we'll be becoming more
> permissive from a correctness point of view.  But we absolutely should
> not try to add entries unilaterally in i915 that haven't been formally
> agreed upon because they may clash with a different definition of the
> row that shows up in the future through formal channels.

-- 
Ville Syrjälä
Intel