On Wednesday 23 December 2009 14:17:32 Pawel Osciak wrote:
> Hello,
> this is the second version of the proposed implementation for mem-to-mem 
> memory
> device framework. Your comments are very welcome.

Hi Pawel,

Thank you for working on this! It's much appreciated. Now I've noticed that
patches regarding memory-to-memory and memory pool tend to get very few 
I suspect that the main reason is that these are SoC-specific features that do
not occur in consumer-type products. So most v4l developers do not have the
interest and motivation (and time!) to look into this.

I'm CC-ing this reply to developers from Intel, TI, Nokia and Renesas in the
hope that they will find some time to review and think about this since this 
affect all of them.

One thing that I am missing is a high-level overview of what we want. Currently
there are patches/RFCs floating around for memory-to-memory support, multiplanar
support and memory-pool support.

What I would like to see is a RFC that ties this all together from the point of
view of the public API. I.e. what are the requirements? Possibly solutions? Open
questions? Forget about how to implement it for the moment, that will follow
from the chosen solutions.

Note that I would suggest though that the memory-pool part is split into two
parts: how to actually allocate the memory is pretty much separate from how v4l
will use it. The actual allocation part is probably quite complex and might
even be hardware dependent and should be discussed separately. But how to use
it is something that can be discussed without needing to know how it was

The lack of discussion in this area does worry me a bit. IMHO this is a very
important area that needs a lot more work. The initiative should be with the
SoC companies and right now it seems only Samsung is active.

BTW, what is the status of the multiplanar RFC? I later realized that that RFC
might be very useful for adding meta-data to buffers. There are several cases
where that is useful: sensors that provide meta-data when capturing a frame and
imagepipelines (particularly in memory-to-memory cases) that want to have all
parameters as part of the meta-data associated with the image. There may well
be more of those.



> In v2.1:
> I am very sorry for the resend, but somehow an orphaned endif found its way to
> Kconfig during the rebase.
> Changes since v1:
> - v4l2_m2m_buf_queue() now requires m2m_ctx as its argument
> - video_queue private data stores driver private data
> - a new submenu in kconfig for mem-to-mem devices
> - minor rebase leftovers cleanup
> A second patch series followed v2 with a new driver for a real device -
> Samsung S3C/S5P image rotator, utilizing this framework.
> This series contains:
> [PATCH v2.1 1/2] V4L: Add memory-to-memory device helper framework for V4L2.
> [PATCH v2.1 2/2] V4L: Add a mem-to-mem V4L2 framework test device.
> [EXAMPLE v2] Mem-to-mem userspace test application.
> Previous discussion and RFC on this topic:
> http://thread.gmane.org/gmane.linux.drivers.video-input-infrastructure/10668
> A mem-to-mem device is a device that uses memory buffers passed by
> userspace applications for both source and destination. This is
> different from existing drivers that use memory buffers for only one
> of those at once.
> In terms of V4L2 such a device would be both of OUTPUT and CAPTURE type.
> Although no such devices are present in the V4L2 framework, a demand for such
> a model exists, e.g. for 'resizer devices'.
> -------------------------------------------------------------------------------
> Mem-to-mem devices
> -------------------------------------------------------------------------------
> In the previous discussion we concluded that we should use one video node with
> two queues, an output (V4L2_BUF_TYPE_VIDEO_OUTPUT) queue for source buffers 
> and
> a capture queue (V4L2_BUF_TYPE_VIDEO_CAPTURE) for destination buffers.
> Each instance has its own set of queues: 2 videobuf_queues, each with a ready
> buffer queue, managed by the framework. Everything is encapsulated in the
> queue context struct:
> struct v4l2_m2m_queue_ctx {
>         struct videobuf_queue   q;
>      /* ... */
>         /* Queue for buffers ready to be processed as soon as this
>          * instance receives access to the device */
>         struct list_head        rdy_queue;
>      /* ... */
> };
> struct v4l2_m2m_ctx {
>      /* ... */
>         /* Capture (output to memory) queue context */
>         struct v4l2_m2m_queue_ctx       cap_q_ctx;
>         /* Output (input from memory) queue context */
>         struct v4l2_m2m_queue_ctx       out_q_ctx;
>      /* ... */
> };
> Streamon can be called for all instances and will not sleep if another 
> instance
> is streaming.
> vidioc_querycap() should report V4L2_CAP_VIDEO_CAPTURE | 
> -------------------------------------------------------------------------------
> Queuing and dequeuing buffers
> -------------------------------------------------------------------------------
> Applications can queue as many buffers as they want and it is not required to
> queue an equal number of source and destination buffers. If there is not 
> enough
> buffers of any type, a new transaction will simply not be scheduled.
> -------------------------------------------------------------------------------
> Source and destination formats
> -------------------------------------------------------------------------------
> Should be set per queue. A helper function to access queues depending on the
> passed type - v4l2_m2m_get_vq() - is supplied. Most of the format-handling 
> code
> is normally located in drivers anyway. The only exception is the "field" 
> member
> of the videobuf_queue struct, which has to be set directly. It breaks
> encapsulation a little bit, but nothing can be done with it.
> -------------------------------------------------------------------------------
> Scheduling
> -------------------------------------------------------------------------------
> Requirements/assumptions:
> 1. More than one instance can be open at the same time.
> 2. Each instance periodically receives exclusive access to the device, 
> performs
> an operation (operations) and yields back the device in a state that allows
> other instances to use it.
> 3. When an instance gets access to the device, it performs a
> "transaction"/"job". A transaction/job is defined as the shortest operation
> that cannot/should not be further divided without having to restart it from
> scratch, or without having to perform expensive reconfiguration of a device,
> etc.
> 4. Transactions can use multiple source/destination buffers.
> 5. Only a driver can tell when it is ready to perform a transaction, so
> a optional callback is provided for that purpose (job_ready()).
> There are three common requirements for a transaction to be ready to run:
> - at least one source buffer ready
> - at least one destination buffer ready
> - streaming on
> - (optional) driver-specific requirements (driver-specific callback function)
> So when buffers are queued by qbuf() or streaming is turned on with
> streamon(), the framework calls v4l2_m2m_try_schedule().
> v4l2_m2m_try_schedule()
> 1. Checks for the above conditions.
> 2. Checks for driver-specific conditions by calling job_ready() callback, if
> supplied.
> 3. If all the checks succeed, it calls v4l2_m2m_schedule() to schedule the
> transaction.
> v4l2_m2m_schedule()
> 1. Checks whether the transaction is already on job queue and schedules it
> if not (by adding it to the job queue).
> 2. Calls v4l2_m2m_try_run().
> v4l2_m2m_try_run()
> 1. Runs a job if and is pending and none is currently running by calling
> device_run() callback.
> When the device_run() callback is called, the driver has to begin the
> transaction. When it is finished, the driver has to call 
> v4l2_m2m_job_finish().
> v4l2_m2m_job_finish()
> 1. Removes the currently running transaction from the job queue and calls
> v4l2_m2m_try_run to (possibly) run the next pending transaction.
> There is also support for forced transaction aborting (when an application
> gets killed). The framework calls job_abort() callback and the driver has
> to abort the transaction as soon as possible and call v4l2_m2m_job_finish()
> to indicate that the transaction has been aborted.
> Additionally, some kind of timeout for transactions could be added to prevent
> instances from claiming the device for too long.
> -------------------------------------------------------------------------------
> Acquiring ready buffers to process
> -------------------------------------------------------------------------------
> Ready buffers can be acquired using v4l2_m2m_next_src_buf()/
> v4l2_m2m_next_dst_buf(). After the transaction they are removed from the 
> queues
> with v4l2_m2m_dst_buf_remove()/v4l2_m2m_src_buf_remove(). This is not
> multi-buffer-transaction-safe. It will have to be modified, but ideally after
> we decide how to handle multi-buffer transactions in videobuf core.
> -------------------------------------------------------------------------------
> poll()
> -------------------------------------------------------------------------------
> We cannot have poll() for multiple queues on one node, so we use poll() for 
> the
> destination queue only.
> -------------------------------------------------------------------------------
> mmap()
> -------------------------------------------------------------------------------
> Requirements:
> - allow mapping buffers from different queues
> - retain "magic" offset values so videobuf can still match buffers by offsets
> The proposed solution involves a querybuf() and mmap() multiplexers:
> a) When a driver calls querybuf(), we have access to the type and we can
> detect which queue to call videobuf_querybuf() on:
>         vq = v4l2_m2m_get_vq(m2m_ctx, buf->type);
>         ret = videobuf_querybuf(vq, buf);
> The offsets returned from videobuf_querybuf() for one of the queues are 
> further
> offset by a predefined constant (DST_QUEUE_OFF_BASE). This way the driver
> (and applications) receive different offsets for the same buffer indexes of
> each queue:
>         if (buf->memory == V4L2_MEMORY_MMAP
>             && vq->type == V4L2_BUF_TYPE_VIDEO_CAPTURE) {
>                 buf->m.offset += DST_QUEUE_OFF_BASE;
>         }
> b) When the application (driver) calls mmap(), the offsets which were modified
> in querybuf() are detected and the proper queue for them chosen based on that.
> Finally, the modified offsets are passed to videobuf_mmap_mapper() for proper
> queues with their offsets changed back to values recognizable by videobuf:
>         unsigned long offset = vma->vm_pgoff << PAGE_SHIFT;
>         struct videobuf_queue *vq;
>         if (offset < DST_QUEUE_OFF_BASE) {
>                 vq = v4l2_m2m_get_src_vq(m2m_ctx);
>         } else {
>                 vq = v4l2_m2m_get_dst_vq(m2m_ctx);
>                 vma->vm_pgoff -= (DST_QUEUE_OFF_BASE >> PAGE_SHIFT);
>         }
>         return videobuf_mmap_mapper(vq, vma);
> -------------------------------------------------------------------------------
> Test device and a userspace application
> -------------------------------------------------------------------------------
> mem2mem_testdev.c is a test driver for the framework. It uses timers for fake
> interrupts and allows testing transaction with different number of buffers
> and transaction durations simultaneously.
> process-vmalloc.c is a capture+output test application for the test device.
> -------------------------------------------------------------------------------
> Future work
> -------------------------------------------------------------------------------
> - read/write support
> - transaction/abort timeouts
> - extracting more common code to the framework? (e.g. per-queue format 
> details,
> transaction length, etc.)
> Best regards
> --
> Pawel Osciak
> Linux Platform Group
> Samsung Poland R&D Center
> --
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Hans Verkuil - video4linux developer - sponsored by TANDBERG
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