drm/vc4: Add T-format scanout support.

The T tiling format is what V3D uses for textures, with no raster
support at all until later revisions of the hardware (and always at a
large 3D performance penalty).  If we can't scan out V3D's format,
then we often need to do a relayout at some stage of the pipeline,
either right before texturing from the scanout buffer (common in X11
without a compositor) or between a tiled screen buffer right before
scanout (an option I've considered in trying to resolve this
inconsistency, but which means needing to use the dirty fb ioctl and
having some update policy).

T-format scanout lets us avoid either of those shadow copies, for a
massive, obvious performance improvement to X11 window dragging
without a compositor.  Unfortunately, enabling a compositor to work
around the discrepancy has turned out to be too costly in memory
consumption for the Raspbian distribution.

Because the HVS operates a scanline at a time, compositing from T does
increase the memory bandwidth cost of scanout.  On my 1920x1080@32bpp
display on a RPi3, we go from about 15% of system memory bandwidth
with linear to about 20% with tiled.  However, for X11 this still ends
up being a huge performance win in active usage.

This patch doesn't yet handle src_x/src_y offsetting within the tiled
buffer.  However, we fail to do so for untiled buffers already.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170608001336.12842-1-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>

1 files changed, 22 insertions, 1 deletions

diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
index 55e301047b3e..7586c46f68bf 100644
--- a/include/uapi/drm/drm_fourcc.h
+++ b/include/uapi/drm/drm_fourcc.h
@@ -182,6 +182,7 @@ extern "C" {
 #define DRM_FORMAT_MOD_VENDOR_SAMSUNG 0x04
 #define DRM_FORMAT_MOD_VENDOR_QCOM    0x05
 #define DRM_FORMAT_MOD_VENDOR_VIVANTE 0x06
+#define DRM_FORMAT_MOD_VENDOR_BROADCOM 0x07
 /* add more to the end as needed */
 
 #define fourcc_mod_code(vendor, val) \
@@ -306,7 +307,6 @@ extern "C" {
  */
 #define DRM_FORMAT_MOD_VIVANTE_SPLIT_SUPER_TILED fourcc_mod_code(VIVANTE, 4)
 
-
 /* NVIDIA Tegra frame buffer modifiers */
 
 /*
@@ -351,6 +351,27 @@ extern "C" {
  */
 #define NV_FORMAT_MOD_TEGRA_16BX2_BLOCK(v) fourcc_mod_tegra_code(2, v)
 
+/*
+ * Broadcom VC4 "T" format
+ *
+ * This is the primary layout that the V3D GPU can texture from (it
+ * can't do linear).  The T format has:
+ *
+ * - 64b utiles of pixels in a raster-order grid according to cpp.  It's 4x4
+ *   pixels at 32 bit depth.
+ *
+ * - 1k subtiles made of a 4x4 raster-order grid of 64b utiles (so usually
+ *   16x16 pixels).
+ *
+ * - 4k tiles made of a 2x2 grid of 1k subtiles (so usually 32x32 pixels).  On
+ *   even 4k tile rows, they're arranged as (BL, TL, TR, BR), and on odd rows
+ *   they're (TR, BR, BL, TL), where bottom left is start of memory.
+ *
+ * - an image made of 4k tiles in rows either left-to-right (even rows of 4k
+ *   tiles) or right-to-left (odd rows of 4k tiles).
+ */
+#define DRM_FORMAT_MOD_BROADCOM_VC4_T_TILED fourcc_mod_code(BROADCOM, 1)
+
 #if defined(__cplusplus)
 }
 #endif