[Intel-xe] [Intel-gfx] [PATCH v2 05/27] drm/i915/xe2lpd: Add fake PCH

[Ville Syrjälä]

On Fri, Sep 08, 2023 at 12:51:09AM -0500, Lucas De Marchi wrote:
> On Fri, Sep 08, 2023 at 08:39:48AM +0300, Ville Syrjälä wrote:
> >On Thu, Sep 07, 2023 at 05:57:19PM -0700, Matt Roper wrote:
> >> On Thu, Sep 07, 2023 at 03:43:59PM -0500, Lucas De Marchi wrote:
> >> > On Thu, Sep 07, 2023 at 10:04:42AM -0700, Matt Roper wrote:
> >> > > On Thu, Sep 07, 2023 at 08:37:35AM -0700, Lucas De Marchi wrote:
> >> > > > From: Gustavo Sousa <gustavo.sousa at intel.com>
> >> > > >
> >> > > > Xe2_LPD has sourth display on the same SOC. As such, define a new fake
> >> > >
> >> > > s/sourth/south/
> >> > >
> >> > > You might also want to drop the word "same" from the description here
> >> > > since NDE and SDE are technically on different dies in this case (NDE is
> >> > > on the compute die, whereas SDE is on the SoC die).  To be 100% accurate
> >> > > we'd want to identify SDE behavior via the PICA's GMD_ID (since PICA
> >> > > also lives on the SoC die for this platform).  But since we've just been
> >> >
> >> > I'd not re-architect this based on where the PICA lives as it seems very
> >> > easy to change in future.... tying the SDE behavior to the PICA behavior
> >> > because they are on the same die, doesn't seem very future proof.
> >>
> >> The point is that tying it to any one thing for every platform is
> >> incorrect; figuring out a) which die is relevant to SDE behavior and b)
> >> how to fingerprint the variant and stepping of that die is very platform
> >> specific.  Art specifically suggested using the PICA ID in cases where
> >> the PICA lives on the die that we need to fingerprint but the NDE does
> >> not.  But again, that's not a silver bullet that can be used on every
> >> single platform.  Nor is using the ISA bus ID like we've done for a long
> >> time.  Nor is using the display version.  Nor is using just the PCI ID.
> >> There's no single answer here, which is why we need a major rethink of
> >> our strategy at some point in the future.  But that overhaul can wait
> >> for a future series; I just want to make sure that the commit messages
> >> here aren't causing further confusion.
> >>
> >> >
> >> > Here the real reason for the change is that from the SW perspective they
> >> > are under the same PCI device and there's no reason to look for a
> >> > different one. Maybe rewording it a "Xe2_LPD has south display on the
> >> > same PCI device" would be simpler?
> >>
> >> No, that would be even less correct; PCI device isn't really related to
> >> any of this.  Obviously at the register level, everything our driver
> >> cares about (NDE, SDE, GT, etc.) is accessed through the same PCI device
> >> (e.g., 00:02.0 on an igpu).  Under the hood the various pieces of that
> >> PCI device (NDE, SDE, render GT, media GT, etc.) might be located
> >> together on a single chip, or may be spread across different dies.  When
> >> spread across different dies, those dies can be mixed-and-matched in
> >> various ways (and it seems like hardware design is trending toward more
> >> flexibility in mix-and-match).
> >>
> >> The register interface to the SDE (i.e., which registers exist and what
> >> bitfields they have inside) hasn't had any meaningful changes in a long
> >> time.  And if it does change in the future, the _interface_ changes are
> >> probably more tied to the display IP version than to anything else.
> >> However there's some important SDE handling that the driver needs to do
> >> that may vary based on the identity of the specific die that's
> >> responsible for doing SDE I/O on a given platform.  I.e., there may be
> >> I/O-related defects+workarounds that require special SDE programming
> >> when a certain die variant and/or stepping is present.  There can also
> >> be differences in how lanes are physically wired up, resulting in pin
> >> mapping changes.  In these cases we need to be able to fingerprint the
> >> identity of the specific die handling the I/O (which might be a compute
> >> die, an SoC die, and IOE die, a PCH die, etc.) and make our decisions
> >> accordingly.  If the SDE I/O happens on the same die as the north
> >> display functionality, then using the display version might be an
> >> effective way to fingerprint.  If the SDE I/O happens on a different die
> >> from the NDE, but on the same die the PICA lives on, the display
> >> architects suggested using the PICA ID in that case.  If neither of
> >> those cases are true, then we may need to look at PCI IDs or something.
> >>
> >> In the past, the PCH was often where the SDE I/O responsibility was so
> >> we needed a way to identify exactly which PCH variant was present.  The
> >> "PCH ID" that we try to match on during driver startup is entirely
> >> unrelated to the SDE; it's just a random bus that we know was always
> >> part of every PCH and always present in the same predictable PCI slot,
> >> so it's handy for identification purposes.  The fact that we're still
> >> looking at the ISA bus on MTL today is 100% wrong because most (maybe
> >> all?) MTL platforms don't even have a PCH (so that ISA bus might be on a
> >> different die that we really don't care about at all).  For MTL I
> >> believe the NDE and the SDE's I/O are both on the same SoC die, so we
> >> should really just be making our decisions based on IP version and/or
> >> graphics device ID.
> >
> >I think ideally SDE would have its own IP version/etc. we could
> >use to identify it.
> 
> maybe some future platform
> 
> >
> >I'm not really sure why we even started down this "fake PCH" route
> >since we never added that for BXT/GLK either, and they managed just
> 
> it was originally done for the discrete cards, I think DG1, and got
> extended to the next ones. Differently than BXT/GLK it doesn't work
> at all to try finding the ISA bridge as that would end up matching the
> wrong one.

BXT/GLK don't look for the ISA bridge either. Well, they do, but
they won't find a matching one and thus we're left with PCH_NONE.

-- 
Ville Syrjälä
Intel