drm/xe/ptl: L3bank mask is not available on the media GT

On PTL platforms with media version 30.00, the fuse registers for
reporting L3 bank availability to the GT just read out as ~0 and do not
provide proper values.  Xe does not use the L3 bank mask for anything
internally; it only passes the mask through to userspace via the GT
topology query.

Since we don't have any way to get the real L3 bank mask, we don't want
to pass garbage to userspace.  Passing a zeroed mask or a copy of the
primary GT's L3 bank mask would also be inaccurate and likely to cause
confusion for userspace.  The best approach is to simply not include L3
in the list of masks returned by the topology query in cases where we
aren't able to provide a meaningful value.  This won't change the
behavior for any existing platforms (where we can always obtain L3 masks
successfully for all GTs), it will only prevent us from mis-reporting
bad information on upcoming platform(s).

There's a good chance this will become a formal workaround in the
future, but for now we don't have a lineage number so "no_media_l3" is
used in place of a lineage as the OOB workaround descriptor.

v2:
 - Re-calculate query size to properly match data returned. (Gustavo)
 - Update kerneldoc to clarify that the L3bank mask may not be included
   in the query results if the hardware doesn't make it available.
   (Gustavo)

Cc: Matt Atwood <matthew.s.atwood@intel.com>
Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Signed-off-by: Shekhar Chauhan <shekhar.chauhan@intel.com>
Co-developed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Acked-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241007154143.2021124-2-matthew.d.roper@intel.com

1 files changed, 3 insertions, 1 deletions

diff --git a/include/uapi/drm/xe_drm.h b/include/uapi/drm/xe_drm.h
index b6fbe4988f2e..c4182e95a619 100644
--- a/include/uapi/drm/xe_drm.h
+++ b/include/uapi/drm/xe_drm.h
@@ -512,7 +512,9 @@ struct drm_xe_query_gt_list {
  *    containing the following in mask:
  *    ``DSS_COMPUTE    ff ff ff ff 00 00 00 00``
  *    means 32 DSS are available for compute.
- *  - %DRM_XE_TOPO_L3_BANK - To query the mask of enabled L3 banks
+ *  - %DRM_XE_TOPO_L3_BANK - To query the mask of enabled L3 banks.  This type
+ *    may be omitted if the driver is unable to query the mask from the
+ *    hardware.
  *  - %DRM_XE_TOPO_EU_PER_DSS - To query the mask of Execution Units (EU)
  *    available per Dual Sub Slices (DSS). For example a query response
  *    containing the following in mask: