Re: [PATCH i-g-t v2] lib/igt_fb/intel: Use correct MOCS for displayable surfaces

Fri, 17 Oct 2025 10:59:07 -0700 


On 17/10/2025 12:29, Ville Syrjälä wrote:

On Fri, Oct 03, 2025 at 02:05:17PM +0100, Tvrtko Ursulin wrote:

Using the uncached MOCS for displayable surfaces is not always correct,
especially when CCS compression is used with which some platforms require
a special uncached entry, otherwise writes get unexpectedly cached.

Lets copy the knowledge of what is the correct MOCS for displayable
surfaces from Mesa and add some new helpers to get it.

Signed-off-by: Tvrtko Ursulin <[email protected]>
Suggested-by: Ville Syrjälä <[email protected]>


The numbers looked all right to me.

Reviewed-by: Ville Syrjälä <[email protected]>


Thanks! I've pushed it and will keep an eye on the results.

There's a bunch of other MOCS stuff under tests/ and I'm not sure
the chosen numbers there make complete sense. Someone might want
to have a good look at those at some point. A few examples:

- kms_ccs uses UC MOCS for some flat CCS stuff even though it
   needs to deal with display coherency, but I'm not sure if there's
   any significant difference between the UC vs. displayable MOCS
   entry on flat CCS platforms

- gem_ccs uses UC even though the data is only accessed by the CPU,
   but then again I'm not sure what the deal is these days with the
   whole LLC/L4 vs. L3 vs. snooping stuff on new platforms



I'll have a brief look in case I spot something obvious. But as flat ccs 
is something I haven't worked with so far, likewise the new platforms, 
it may not be a very effective look.


Regards,

Tvrtko

---
v2:
  * Renamed external_index to displayable_index. (Ville)
---
  lib/igt_fb.c       |  2 +-
  lib/intel_bufops.c |  2 ++
  lib/intel_mocs.c   | 21 +++++++++++++++++++--
  lib/intel_mocs.h   |  2 ++
  4 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/lib/igt_fb.c b/lib/igt_fb.c
index 03ede3a6fa20..b5a16f9cbe90 100644
--- a/lib/igt_fb.c
+++ b/lib/igt_fb.c
@@ -2712,7 +2712,7 @@ igt_fb_create_intel_buf(int fd, struct buf_ops *bops,
                                    fb->strides[0],
                                    region,
                                    intel_get_pat_idx_uc(fd),
-                                   DEFAULT_MOCS_INDEX);
+                                   DISPLAYABLE_MOCS_INDEX);
        intel_buf_set_name(buf, name);

  
  	/* only really needed for proper CCS handling */

diff --git a/lib/intel_bufops.c b/lib/intel_bufops.c
index 475b0d1f7b10..1196069a500f 100644
--- a/lib/intel_bufops.c
+++ b/lib/intel_bufops.c
@@ -1008,6 +1008,8 @@ static void __intel_buf_init(struct buf_ops *bops,
        buf->pat_index = pat_index;
        if (mocs_index == DEFAULT_MOCS_INDEX)
                mocs_index = intel_get_uc_mocs_index(bops->fd);
+       else if (mocs_index == DISPLAYABLE_MOCS_INDEX)
+               mocs_index = intel_get_displayable_mocs_index(bops->fd);
        buf->mocs_index = mocs_index;
        IGT_INIT_LIST_HEAD(&buf->link);

  
diff --git a/lib/intel_mocs.c b/lib/intel_mocs.c

index e0c33c31c088..9809e32113eb 100644
--- a/lib/intel_mocs.c
+++ b/lib/intel_mocs.c
@@ -9,12 +9,14 @@
  struct drm_intel_mocs_index {
        uint8_t uc_index;
        uint8_t wb_index;
+       uint8_t displayable_index;
        uint8_t defer_to_pat_index;
  };

  
  static void get_mocs_index(int fd, struct drm_intel_mocs_index *mocs)

  {
        uint16_t devid = intel_get_drm_devid(fd);
+       unsigned int ip_ver = intel_graphics_ver(devid);

  
  	/*

         * Gen >= 12 onwards don't have a setting for PTE,
@@ -23,25 +25,31 @@ static void get_mocs_index(int fd, struct 
drm_intel_mocs_index *mocs)
         * This helper function is providing current UC as well
         * as WB MOCS index based on platform.
         */
-       if (intel_graphics_ver(devid) >= IP_VER(20, 0)) {
+       if (ip_ver >= IP_VER(20, 0)) {
                mocs->uc_index = 3;
                mocs->wb_index = 4;
+               mocs->displayable_index = 1;
                mocs->defer_to_pat_index = 0;
        } else if (IS_METEORLAKE(devid)) {
                mocs->uc_index = 5;
                mocs->wb_index = 1;
+               mocs->displayable_index = 14;
        } else if (IS_DG2(devid)) {
                mocs->uc_index = 1;
                mocs->wb_index = 3;
+               mocs->displayable_index = 3;
        } else if (IS_DG1(devid)) {
                mocs->uc_index = 1;
                mocs->wb_index = 5;
-       } else if (IS_GEN12(devid)) {
+               mocs->displayable_index = 5;
+       } else if (ip_ver >= IP_VER(12, 0)) {
                mocs->uc_index = 3;
                mocs->wb_index = 2;
+               mocs->displayable_index = 61;
        } else {
                mocs->uc_index = I915_MOCS_PTE;
                mocs->wb_index = I915_MOCS_CACHED;
+               mocs->displayable_index = I915_MOCS_PTE;
        }
  }

  
@@ -63,6 +71,15 @@ uint8_t intel_get_uc_mocs_index(int fd)

        return mocs.uc_index;
  }

  
+uint8_t intel_get_displayable_mocs_index(int fd)

+{
+       struct drm_intel_mocs_index mocs;
+
+       get_mocs_index(fd, &mocs);
+
+       return mocs.displayable_index;
+}
+
  uint8_t intel_get_defer_to_pat_mocs_index(int fd)
  {
        struct drm_intel_mocs_index mocs;
diff --git a/lib/intel_mocs.h b/lib/intel_mocs.h
index 8597286d259d..394bb41be042 100644
--- a/lib/intel_mocs.h
+++ b/lib/intel_mocs.h
@@ -9,9 +9,11 @@
  #include <stdint.h>

  
  #define DEFAULT_MOCS_INDEX ((uint8_t)-1)

+#define DISPLAYABLE_MOCS_INDEX ((uint8_t)-2)

  
  uint8_t intel_get_wb_mocs_index(int fd);

  uint8_t intel_get_uc_mocs_index(int fd);
+uint8_t intel_get_displayable_mocs_index(int fd);
  uint8_t intel_get_defer_to_pat_mocs_index(int fd);

  
  #endif /* _INTEL_MOCS_H */

--
2.48.0


























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