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

[Tvrtko Ursulin]

On 09/10/2025 15:12, Kamil Konieczny wrote:
Hi Tvrtko,
On 2025-10-03 at 14:05:17 +0100, Tvrtko Ursulin wrote:

small nit, as this is touching more than one lib, you could
just write it short:

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

No need for resend unless you will send v3 or want to retest it
again.

I think I started with a smaller patch and forgot to change the tags 
when I refactored it.

Anyway, CI looks good AFAICT so we could merge it if everyone agrees?

I would then need to send the rebased xe series to see if it is 
completely stable without the clflush patch.

Regards,

Tvrtko

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 <tvrtko.ursu...@igalia.com>
Suggested-by: Ville Syrjälä <ville.syrj...@linux.intel.com>
---
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