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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>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.