172 lines
11 KiB
ReStructuredText
172 lines
11 KiB
ReStructuredText
Single-sampled Color Compression
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================================
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Starting with Ivy Bridge, Intel graphics hardware provides a form of color
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compression for single-sampled surfaces. In its initial form, this provided an
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acceleration of render target clear operations that, in the common case, allows
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you to avoid almost all of the bandwidth of a full-surface clear operation. On
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Sky Lake, single-sampled color compression was extended to allow for the
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compression color values from actual rendering and not just the initial clear.
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From here on, the older Ivy Bridge form of color compression will be called
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"fast-clears" and term "color compression" will be reserved for the more
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powerful Sky Lake form.
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The documentation for Ivy Bridge through Broadwell overloads the term MCS for
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referring both to the *multisample control surface* used for multisample
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compression and the control surface used for fast-clears. In ISL, the
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:cpp:enumerator:`isl_aux_usage::ISL_AUX_USAGE_MCS` enum always refers to
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multisample color compression while the
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:cpp:enumerator:`isl_aux_usage::ISL_AUX_USAGE_CCS_` enums always refer to
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single-sampled color compression. Throughout this chapter and the rest of the
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ISL documentation, we will use the term "color control surface", abbreviated
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CCS, to denote the control surface used for both fast-clears and color
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compression. While this is still an overloaded term, Ivy Bridge fast-clears
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are much closer to Sky Lake color compression than they are to multisample
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compression.
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CCS data
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--------
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Fast clears and CCS are possibly the single most poorly documented aspect of
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surface layout/setup for Intel graphics hardware (with HiZ coming in a neat
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second). All the documentation really says is that you can use an MCS buffer on
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single-sampled surfaces (we will call it the CCS in this case). It also
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provides some documentation on how to program the hardware to perform clear
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operations, but that's it. How big is this buffer? What does it contain?
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Those question are left as exercises to the reader. Almost everything we know
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about the contents of the CCS is gleaned from reverse-engineering of the
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hardware. The best bit of documentation we have ever had comes from the
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display section of the Sky Lake PRM Vol 12 section on planes (p. 159):
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The Color Control Surface (CCS) contains the compression status of the
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cache-line pairs. The compression state of the cache-line pair is
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specified by 2 bits in the CCS. Each CCS cache-line represents an area
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on the main surface of 16x16 sets of 128 byte Y-tiled cache-line-pairs.
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CCS is always Y tiled.
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While this is technically for color compression and not fast-clears, it
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provides a good bit of insight into how color compression and fast-clears
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operate. Each cache-line pair, in the main surface corresponds to 1 or 2 bits
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in the CCS. The primary difference, as far as the current discussion is
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concerned, is that fast-clears use only 1 bit per cache-line pair whereas color
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compression uses 2 bits.
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What is a cache-line pair? Both the X and Y tiling formats are arranged as an
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8x8 grid of cache lines. (See the :doc:`chapter on tiling <tiling>` for more
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details.) In either case, a cache-line pair is a pair of cache lines whose
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starting addresses differ by 512 bytes or 8 cache lines. This results in the
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two cache lines being vertically adjacent when the main surface is X-tiled and
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horizontally adjacent when the main surface is Y-tiled. For an X-tiled surface
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this forms an area of 64B x 2rows and for a Y-tiled surface this forms an area
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of 32B x 4rows. In either case, it is guaranteed that, regardless of surface
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format, each 2x2 subspan coming out of a shader will land entirely within one
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cache-line pair.
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What is the correspondence between bits and cache-line pairs? The best model I
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(Jason) know of is to consider the CCS as having a 1-bit color format for
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fast-clears and a 2-bit format for color compression and a special tiling
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format. The CCS tiling formats operate on a 1 or 2-bit granularity rather than
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the byte granularity of most tiling formats.
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The following table represents the bit-layouts that yield the CCS tiling format
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on different hardware generations. Bits 0-11 correspond to the regular swizzle
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of bytes within a 4KB page whereas the negative bits represent the address of
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the particular 1 or 2-bit portion of a byte. (Note: The haswell data was
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gathered on a dual-channel system so bit-6 swizzling was enabled. It's unclear
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how this affects the CCS layout.)
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============ ======== =========== =========== ====================== =========== =========== =========== =========== =========== =========== =========== =========== =========== =========== =========== ===========
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Generation Tiling 11 10 9 8 7 6 5 4 3 2 1 0 -1 -2 -3
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============ ======== =========== =========== ====================== =========== =========== =========== =========== =========== =========== =========== =========== =========== =========== =========== ===========
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Ivy Bridge X or Y :math:`u_6` :math:`u_5` :math:`u_4` :math:`v_7` :math:`v_6` :math:`v_5` :math:`v_4` :math:`v_2` :math:`v_3` :math:`v_1` :math:`v_0` :math:`u_3` :math:`u_2` :math:`u_1` :math:`u_0`
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Haswell X :math:`u_6` :math:`u_5` :math:`v_3 \oplus u_1` :math:`v_7` :math:`v_6` :math:`v_5` :math:`v_4` :math:`v_2` :math:`v_3` :math:`v_1` :math:`v_0` :math:`u_4` :math:`u_3` :math:`u_2` :math:`u_0`
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Haswell Y :math:`u_6` :math:`u_5` :math:`v_2 \oplus u_1` :math:`v_7` :math:`v_6` :math:`v_5` :math:`v_4` :math:`v_2` :math:`v_3` :math:`v_1` :math:`v_0` :math:`u_4` :math:`u_3` :math:`u_2` :math:`u_0`
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Broadwell X :math:`u_6` :math:`u_5` :math:`u_4` :math:`v_7` :math:`v_6` :math:`v_5` :math:`v_4` :math:`u_3` :math:`v_3` :math:`u_2` :math:`u_1` :math:`u_0` :math:`v_2` :math:`v_1` :math:`v_0`
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Broadwell Y :math:`u_6` :math:`u_5` :math:`u_4` :math:`v_7` :math:`v_6` :math:`v_5` :math:`v_4` :math:`v_2` :math:`v_3` :math:`u_3` :math:`u_2` :math:`u_1` :math:`v_1` :math:`v_0` :math:`u_0`
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Sky Lake Y :math:`u_6` :math:`u_5` :math:`u_4` :math:`v_6` :math:`v_5` :math:`v_4` :math:`v_3` :math:`v_2` :math:`v_1` :math:`u_3` :math:`u_2` :math:`u_1` :math:`v_0` :math:`u_0`
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============ ======== =========== =========== ====================== =========== =========== =========== =========== =========== =========== =========== =========== =========== =========== =========== ===========
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CCS surface layout
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------------------
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Starting with Broadwell, fast-clears and color compression can be used on
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mipmapped and array surfaces. When considered from a higher level, the CCS is
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layed out like any other surface. The Broadwell and Sky Lake PRMs describe
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this as follows:
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Broadwell PRM Vol 7, "MCS Buffer for Render Target(s)" (p. 676):
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Mip-mapped and arrayed surfaces are supported with MCS buffer layout with
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these alignments in the RT space: Horizontal Alignment = 256 and Vertical
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Alignment = 128.
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Broadwell PRM Vol 2d, "RENDER_SURFACE_STATE" (p. 279):
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For non-multisampled render target's auxiliary surface, MCS, QPitch must be
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computed with Horizontal Alignment = 256 and Surface Vertical Alignment =
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128. These alignments are only for MCS buffer and not for associated render
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target.
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Sky Lake PRM Vol 7, "MCS Buffer for Render Target(s)" (p. 632):
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Mip-mapped and arrayed surfaces are supported with MCS buffer layout with
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these alignments in the RT space: Horizontal Alignment = 128 and Vertical
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Alignment = 64.
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Sky Lake PRM Vol. 2d, "RENDER_SURFACE_STATE" (p. 435):
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For non-multisampled render target's CCS auxiliary surface, QPitch must be
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computed with Horizontal Alignment = 128 and Surface Vertical Alignment
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= 256. These alignments are only for CCS buffer and not for associated
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render target.
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Empirical evidence seems to confirm this. On Sky Lake, the vertical alignment
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is always one cache line. The horizontal alignment, however, varies by main
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surface format: 1 cache line for 32bpp, 2 for 64bpp and 4 cache lines for
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128bpp formats. This nicely corresponds to the alignment of 128x64 pixels in
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the primary color surface. The second PRM citation about Sky Lake CCS above
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gives a vertical alignment of 256 rather than 64. With a little
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experimentation, this additional alignment appears to only apply to QPitch and
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not to the miplevels within a slice.
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On Broadwell, each miplevel in the CCS is aligned to a cache-line pair
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boundary: horizontal when the primary surface is X-tiled and vertical when
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Y-tiled. For a 32bpp format, this works out to an alignment of 256x128 main
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surface pixels regardless of X or Y tiling. On Sky Lake, the alignment is
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a single cache line which works out to an alignment of 128x64 main surface
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pixels.
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TODO: More than just 32bpp formats on Broadwell!
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Once armed with the above alignment information, we can lay out the CCS surface
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itself. The way ISL does CCS layout calculations is by a very careful and
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subtle application of its normal surface layout code.
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Above, we described the CCS data layout as mapping of address bits. In
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ISL, this is represented by :cpp:enumerator:`isl_tiling::ISL_TILING_CCS`. The
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logical and physical tile dimensions corresponding to the above mapping.
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We also have special :cpp:enum:`isl_format` enums for CCS. These formats are 1
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bit-per-pixel on Ivy Bridge through Broadwell and 2 bits-per-pixel on Skylake
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and above to correspond to the 1 and 2-bit values represented in the CCS data.
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They have a block size (similar to a block compressed format such as BC or
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ASTC) which says what area (in surface elements) in the main surface is covered
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by a single CCS element (1 or 2-bit). Because this depends on the main surface
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tiling and format, we have several different CCS formats.
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Once the appropriate :cpp:enum:`isl_format` has been selected, computing the
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size and layout of a CCS surface is as simple as passing the same surface
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creation parameters to :cpp:func:`isl_surf_init_s` as were used to create the
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primary surface only with :cpp:enumerator:`isl_tiling::ISL_TILING_CCS` and the
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correct CCS format. This not only results in a correctly sized surface but
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most other ISL helpers for things such as computing offsets into surfaces work
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correctly as well.
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CCS on Tigerlake and above
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--------------------------
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Starting with Tigerlake, CCS is no longer done via a surface and, instead, the
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term CCS gets overloaded once again (gotta love it!) to now refer to a form of
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universal compression which can be applied to almost any surface. Nothing in
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this chapter applies to any hardware with a graphics IP version 12 or above.
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