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Userspace is faced with a dilemma. The kernel requires implicit fencing to manage resource usage (we always must wait for the GPU to finish before releasing its PTE) and for third parties. However, userspace may wish to avoid this serialisation if it is either using explicit fencing between parties and wants more fine-grained access to buffers (e.g. it may partition the buffer between uses and track fences on ranges rather than the implicit fences tracking the whole object). It follows that userspace needs a mechanism to avoid the kernel's serialisation on its implicit fences before execbuf execution. The next question is whether this is an object, execbuf or context flag. Hybrid users (such as using explicit EGL_ANDROID_native_sync fencing on shared winsys buffers, but implicit fencing on internal surfaces) require a per-object level flag. Given that this flag need to be only set once for the lifetime of the object, this reduces the convenience of having an execbuf or context level flag (and avoids having multiple pieces of uABI controlling the same feature). Incorrect use of this flag will result in rendering corruption and GPU hangs - but will not result in use-after-free or similar resource tracking issues. Serious caveat: write ordering is not strictly correct after setting this flag on a render target on multiple engines. This affects all subsequent GEM operations (execbuf, set-domain, pread) and shared dma-buf operations. A fix is possible - but costly (both in terms of further ABI changes and runtime overhead). Testcase: igt/gem_exec_async Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Acked-by: Chad Versace <chadversary@chromium.org> Link: http://patchwork.freedesktop.org/patch/msgid/20170127094008.27489-1-chris@chris-wilson.co.uk |
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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.