Hello all, Here's a set of patches to implement support for the request API that has been discussed many, many times over the past few years. Some of the patches are my own while some have been written by Laurent and Hans. I've made a few changes to these patches and added more, mainly: - Implement request state in the framework in order to prevent performing invalid actions on requests, e.g. deleting a queued request or queueing a request which is already queued. - Use 32 bits for the request ID. As requests are global, a 16-bit space could become a limitation. The IDs are reserved using ida (as in linux/idr.h) instead of a counter. This way we can ensure that once the counter wraps around, we will continue to allocate unique IDs for new requests. - Add media events to signal applications on completed events. Only the file handle which queued the request is notified. I've chosen this instead of a request specific means to dequeue events as there isn't really anything to obtain other than the information that the request has been completed: there's no data related to it, no buffers as in V4L2. Some applications may not be interested in receiving such events so the events are optional, specific to the queued request. - Add poll support to tell the user there are dequeueable events. My own use case for this is a little bit more limited than for some others, i.e. I only need to bind video buffers to requests. That is currently functional with these patches. My open questions and to-do items I'm aware of: - Global ID space vs. file handles. Should requests be referred to by global IDs or file handles specific to a process? V4L2 uses IDs but V4L2 devices (other than m2m) can meaningfully be used only by a single program (or co-operative programs) at a time whereas the media device could conceivably be accessed and used for different purposes by multiple programs at the same time as the resources accessible through the media device are numerous and often independent of each other. A badly behaving application could disturb other applications using the same media device even if no resource conflict exist by accessing the same request IDs than other applications. - Request completion and deletion. Should completed requests be deleted automatically, or should the request return to an "empty" state after completion? Applications typically have to create a new request right after a completion of an earlier one, and sparing one additional IOCTL call would be nice. (In current implementation the requests are deleted in completion, but this would be a trivial change.) - Video buffers vs. requests. In the current implementation, I'm using VIDIOC_QBUF() from user space to associate buffers with requests. This is convenient in drivers since the only extra steps to support requests (vs. operation without requests) is to find the request and not really queueing the buffer yet. What's noteworthy as well that the VB2 buffer is in correct state after this when the request is queued. - Subsystem framework specific request information. The requests are about the media device, and struct media_device_request is free of e.g. V4L2 related information. Adding a pointer to V4L2 related data would make it easier to add request handling related functionality to the V4L2 framework. - MEDIA_IOC_REQUEST_CMD + (ALLOC/DELETE/QUEUE/APPLY) vs. MEDIA_IOC_REQUEST_(ALLOC/DELETE/QUEUE/APPLY). Should we continue to have a single IOCTL on the media device for requests, or should each operation on a request have a distinct IOCTL? The current implementation has a single IOCTL. - VB2 queues are quite self-sufficient at the moment. Starting start in a queue requires at least one queued buffer whereas a stream containing multiple queues using requests could start e.g. by having a single buffer in a request for three streaming queues. The functionality would need to be relaxed to properly support requests. - Limit number of allocated requests and dequeueable events to prevent unintentional or intentional system memory exhaustion. The patchset is dependent on two patchsets (Media device IOCTL handling refactoring and Media device file handle support) I've posted lately: <URL:http://www.spinics.net/lists/linux-media/msg100194.html> <URL:http://www.spinics.net/lists/linux-media/msg100188.html> The code can be found here in the branch called "request": <URL:http://git.retiisi.org.uk/?p=~sailus/linux.git;a=summary> I intend to provide a test program for testing requests in the near future. Questions and comments are the most welcome! -- Kind regards, Sakari -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
