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On architectures where there is no coherent caching such as ARM use the dma_alloc_noncontiguous API and handle manually the cache flushing using dma_sync_sgtable(). If the architechture has coherent cache, the API falls back to alloc_dma_pages, so we can remove the coherent caching code-path from the driver, making it simpler. With this patch on the affected architectures we can measure up to 20x performance improvement in uvc_video_copy_data_work(). Eg: aarch64 with an external usb camera NON_CONTIGUOUS frames: 999 packets: 999 empty: 0 (0 %) errors: 0 invalid: 0 pts: 0 early, 0 initial, 999 ok scr: 0 count ok, 0 diff ok sof: 2048 <= sof <= 0, freq 0.000 kHz bytes 67034480 : duration 33303 FPS: 29.99 URB: 523446/4993 uS/qty: 104.836 avg 132.532 std 13.230 min 831.094 max (uS) header: 76564/4993 uS/qty: 15.334 avg 15.229 std 3.438 min 186.875 max (uS) latency: 468945/4992 uS/qty: 93.939 avg 132.577 std 9.531 min 824.010 max (uS) decode: 54161/4993 uS/qty: 10.847 avg 6.313 std 1.614 min 111.458 max (uS) raw decode speed: 9.931 Gbits/s raw URB handling speed: 1.025 Gbits/s throughput: 16.102 Mbits/s URB decode CPU usage 0.162600 % COHERENT frames: 999 packets: 999 empty: 0 (0 %) errors: 0 invalid: 0 pts: 0 early, 0 initial, 999 ok scr: 0 count ok, 0 diff ok sof: 2048 <= sof <= 0, freq 0.000 kHz bytes 54683536 : duration 33302 FPS: 29.99 URB: 1478135/4000 uS/qty: 369.533 avg 390.357 std 22.968 min 3337.865 max (uS) header: 79761/4000 uS/qty: 19.940 avg 18.495 std 1.875 min 336.719 max (uS) latency: 281077/4000 uS/qty: 70.269 avg 83.102 std 5.104 min 735.000 max (uS) decode: 1197057/4000 uS/qty: 299.264 avg 318.080 std 1.615 min 2806.667 max (uS) raw decode speed: 365.470 Mbits/s raw URB handling speed: 295.986 Mbits/s throughput: 13.136 Mbits/s URB decode CPU usage 3.594500 % In non-affected architectures we see no significant impact. Eg: x86 with an external usb camera NON_CONTIGUOUS frames: 999 packets: 999 empty: 0 (0 %) errors: 0 invalid: 0 pts: 0 early, 0 initial, 999 ok scr: 0 count ok, 0 diff ok sof: 2048 <= sof <= 0, freq 0.000 kHz bytes 70179056 : duration 33301 FPS: 29.99 URB: 288901/4897 uS/qty: 58.995 avg 26.022 std 4.319 min 253.853 max (uS) header: 54792/4897 uS/qty: 11.189 avg 6.218 std 0.620 min 61.750 max (uS) latency: 236602/4897 uS/qty: 48.315 avg 24.244 std 1.764 min 240.924 max (uS) decode: 52298/4897 uS/qty: 10.679 avg 8.299 std 1.638 min 108.861 max (uS) raw decode speed: 10.796 Gbits/s raw URB handling speed: 1.949 Gbits/s throughput: 16.859 Mbits/s URB decode CPU usage 0.157000 % COHERENT frames: 999 packets: 999 empty: 0 (0 %) errors: 0 invalid: 0 pts: 0 early, 0 initial, 999 ok scr: 0 count ok, 0 diff ok sof: 2048 <= sof <= 0, freq 0.000 kHz bytes 71818320 : duration 33301 FPS: 29.99 URB: 321021/5000 uS/qty: 64.204 avg 23.001 std 10.430 min 268.837 max (uS) header: 54308/5000 uS/qty: 10.861 avg 5.104 std 0.778 min 54.736 max (uS) latency: 268799/5000 uS/qty: 53.759 avg 21.827 std 6.095 min 255.153 max (uS) decode: 52222/5000 uS/qty: 10.444 avg 7.137 std 1.874 min 71.103 max (uS) raw decode speed: 11.048 Gbits/s raw URB handling speed: 1.789 Gbits/s throughput: 17.253 Mbits/s URB decode CPU usage 0.156800 % Signed-off-by: Ricardo Ribalda <ribalda@chromium.org> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Tomasz Figa <tfiga@chromium.org> Signed-off-by: Christoph Hellwig <hch@lst.de>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
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.