[Intel-gfx] [PATCH v4 15/16] drm/i915/Flat-CCS: Document on Flat-CCS memory compression

[Ramalingam C]

Documents the Flat-CCS feature and kernel handling required along with
modifiers used.

Signed-off-by: Ramalingam C <ramalingam.c at intel.com>
cc: Simon Ser <contact at emersion.fr>
cc: Pekka Paalanen <ppaalanen at gmail.com>
Cc: Jordan Justen <jordan.l.justen at intel.com>
Cc: Kenneth Graunke <kenneth at whitecape.org>
Cc: mesa-dev at lists.freedesktop.org
Cc: Tony Ye <tony.ye at intel.com>
Cc: Slawomir Milczarek <slawomir.milczarek at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_migrate.c | 47 +++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/drivers/gpu/drm/i915/gt/intel_migrate.c b/drivers/gpu/drm/i915/gt/intel_migrate.c
index 0fb83d0bec91..2d7ea9b6e8fb 100644
--- a/drivers/gpu/drm/i915/gt/intel_migrate.c
+++ b/drivers/gpu/drm/i915/gt/intel_migrate.c
@@ -595,6 +595,53 @@ intel_context_migrate_copy(struct intel_context *ce,
 	return err;
 }
 
+/**
+ * DOC: Flat-CCS - Memory compression for Local memory
+ *
+ * On Xe-HP and later devices, we use dedicated compression control state (CCS)
+ * stored in local memory for each surface, to support the 3D and media
+ * compression formats.
+ *
+ * The memory required for the CCS of the entire local memory is 1/256 of the
+ * local memory size. So before the kernel boot, the required memory is reserved
+ * for the CCS data and a secure register will be programmed with the CCS base
+ * address.
+ *
+ * Flat CCS data needs to be cleared when a lmem object is allocated.
+ * And CCS data can be copied in and out of CCS region through
+ * XY_CTRL_SURF_COPY_BLT. CPU can't access the CCS data directly.
+ *
+ * When we exaust the lmem, if the object's placements support smem, then we can
+ * directly decompress the compressed lmem object into smem and start using it
+ * from smem itself.
+ *
+ * But when we need to swapout the compressed lmem object into a smem region
+ * though objects' placement doesn't support smem, then we copy the lmem content
+ * as it is into smem region along with ccs data (using XY_CTRL_SURF_COPY_BLT).
+ * When the object is referred, lmem content will be swaped in along with
+ * restoration of the CCS data (using XY_CTRL_SURF_COPY_BLT) at corresponding
+ * location.
+ *
+ *
+ * Flat-CCS Modifiers for different compression formats
+ * ----------------------------------------------------
+ *
+ * I915_FORMAT_MOD_F_TILED_DG2_RC_CCS - used to indicate the buffers of Flat CCS
+ * render compression formats. Though the general layout is same as
+ * I915_FORMAT_MOD_Y_TILED_GEN12_RC_CCS, new hashing/compression algorithm is
+ * used. Render compression uses 128 byte compression blocks
+ *
+ * I915_FORMAT_MOD_F_TILED_DG2_MC_CCS -used to indicate the buffers of Flat CCS
+ * media compression formats. Though the general layout is same as
+ * I915_FORMAT_MOD_Y_TILED_GEN12_MC_CCS, new hashing/compression algorithm is
+ * used. Media compression uses 256 byte compression blocks.
+ *
+ * I915_FORMAT_MOD_F_TILED_DG2_RC_CCS_CC - used to indicate the buffers of Flat
+ * CCS clear color render compression formats. Unified compression format for
+ * clear color render compression. The genral layout is a tiled layout using
+ * 4Kb tiles i.e Tile4 layout.
+ */
+
 static inline u32 *i915_flush_dw(u32 *cmd, u64 dst, u32 flags)
 {
 	/* Mask the 3 LSB to use the PPGTT address space */
-- 
2.20.1