1=================== 2Sync File API Guide 3=================== 4 5:Author: Gustavo Padovan <gustavo at padovan dot org> 6 7This document serves as a guide for device drivers writers on what the 8sync_file API is, and how drivers can support it. Sync file is the carrier of 9the fences(struct dma_fence) that are needed to synchronize between drivers or 10across process boundaries. 11 12The sync_file API is meant to be used to send and receive fence information 13to/from userspace. It enables userspace to do explicit fencing, where instead 14of attaching a fence to the buffer a producer driver (such as a GPU or V4L 15driver) sends the fence related to the buffer to userspace via a sync_file. 16 17The sync_file then can be sent to the consumer (DRM driver for example), that 18will not use the buffer for anything before the fence(s) signals, i.e., the 19driver that issued the fence is not using/processing the buffer anymore, so it 20signals that the buffer is ready to use. And vice-versa for the consumer -> 21producer part of the cycle. 22 23Sync files allows userspace awareness on buffer sharing synchronization between 24drivers. 25 26Sync file was originally added in the Android kernel but current Linux Desktop 27can benefit a lot from it. 28 29in-fences and out-fences 30------------------------ 31 32Sync files can go either to or from userspace. When a sync_file is sent from 33the driver to userspace we call the fences it contains 'out-fences'. They are 34related to a buffer that the driver is processing or is going to process, so 35the driver creates an out-fence to be able to notify, through 36dma_fence_signal(), when it has finished using (or processing) that buffer. 37Out-fences are fences that the driver creates. 38 39On the other hand if the driver receives fence(s) through a sync_file from 40userspace we call these fence(s) 'in-fences'. Receiving in-fences means that 41we need to wait for the fence(s) to signal before using any buffer related to 42the in-fences. 43 44Creating Sync Files 45------------------- 46 47When a driver needs to send an out-fence userspace it creates a sync_file. 48 49Interface:: 50 51 struct sync_file *sync_file_create(struct dma_fence *fence); 52 53The caller pass the out-fence and gets back the sync_file. That is just the 54first step, next it needs to install an fd on sync_file->file. So it gets an 55fd:: 56 57 fd = get_unused_fd_flags(O_CLOEXEC); 58 59and installs it on sync_file->file:: 60 61 fd_install(fd, sync_file->file); 62 63The sync_file fd now can be sent to userspace. 64 65If the creation process fail, or the sync_file needs to be released by any 66other reason fput(sync_file->file) should be used. 67 68Receiving Sync Files from Userspace 69----------------------------------- 70 71When userspace needs to send an in-fence to the driver it passes file descriptor 72of the Sync File to the kernel. The kernel can then retrieve the fences 73from it. 74 75Interface:: 76 77 struct dma_fence *sync_file_get_fence(int fd); 78 79 80The returned reference is owned by the caller and must be disposed of 81afterwards using dma_fence_put(). In case of error, a NULL is returned instead. 82 83References: 84 851. struct sync_file in include/linux/sync_file.h 862. All interfaces mentioned above defined in include/linux/sync_file.h 87