Hi Takashi,
On 19.10.2023 03:07, Takashi Iwai wrote:
On Wed, 18 Oct 2023 12:48:23 +0200, Matias Ezequiel Vara Larsen wrote:
This commit replaces the mmap mechanism with the copy() and fill_silence() callbacks for both capturing and playback for the virtio-sound driver. This change is required to prevent the updating of the content of a buffer that is already in the available ring.
The current mechanism splits a dma buffer into descriptors that are exposed to the device. This dma buffer is shared with the user application. When the device consumes a buffer, the driver moves the request from the used ring to available ring.
The driver exposes the buffer to the device without knowing if the content has been updated from the user. The section 2.8.21.1 of the virtio spec states that: "The device MAY access the descriptor chains the driver created and the memory they refer to immediately". If the device picks up buffers from the available ring just after it is notified, it happens that the content may be old.
By providing the copy() callback, the driver first updates the content of the buffer, and then, exposes the buffer to the device by enqueuing it in the available ring. Thus, device always picks up a buffer that is updated. During copy(), the number of requests enqueued depends on the "pos" and "bytes" arguments. The length of each request is period_size bytes.
For capturing, the driver starts by exposing all the available buffers to device. After device updates the content of a buffer, it enqueues it in the used ring. It is only after the copy() for capturing is issued that the driver re-enqueues the buffer in the available ring.
Co-developed-by: Anton Yakovlev anton.yakovlev@opensynergy.com Signed-off-by: Matias Ezequiel Vara Larsen mvaralar@redhat.com
Changelog: v1 -> v2:
- Use snd_pcm_set_managed_buffer_all()for buffer allocation/freeing.
- Make virtsnd_pcm_msg_send() generic by specifying the offset and size for the modified part of the buffer; this way no assumptions need to be made.
- Disable SNDRV_PCM_INFO_NO_REWINDS since now only sequential reading/writing of frames is supported.
- Correct comment at virtsnd_pcm_msg_send().
- v1 patch at: https://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3...
sound/virtio/virtio_pcm.c | 7 ++- sound/virtio/virtio_pcm.h | 9 ++-- sound/virtio/virtio_pcm_msg.c | 93 ++++++++++++++++++++++------------- sound/virtio/virtio_pcm_ops.c | 81 +++++++++++++++++++++++++----- 4 files changed, 137 insertions(+), 53 deletions(-)
Most of the code changes look good, but I wonder:
diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c index c10d91fff2fb..66d67eef1bcc 100644 --- a/sound/virtio/virtio_pcm.c +++ b/sound/virtio/virtio_pcm.c @@ -104,12 +104,11 @@ static int virtsnd_pcm_build_hw(struct virtio_pcm_substream *vss, * only message-based transport. */ vss->hw.info =
SNDRV_PCM_INFO_MMAP |SNDRV_PCM_INFO_MMAP_VALID |Do we need the removal of those MMAP features inevitably? Usually mmap can still work even if the driver implements the copy ops. Those aren't always mutual exclusive.
The driver uses a message queue to communicate with the device. Thus, the audio buffer is sliced into several I/O requests (= number of periods) of the same size (= period size).
Before this, all such requests were enqueued when the substream started, and immediately re-enqueued once the request is completed. This approach made it possible to add mmap support. But for mmap there are no explicit notifications from the application how many frames were written or read. Thus, it was assumed that the virtual device should read/write frames to requests based on timings. And there are some problems here:
1. This was found to violate the virtio specification: if a request is already in the queue, the device can safely read/write there at any time. 2. It looks like this breaks the use case with swiotlb. Personally I'm not sure how the application handles DMA ownership in the case of mmaped buffer.
To correctly implement mmap support, instead of transferring data via a message queue, the driver and device must have a shared memory region. We can add mmap in the future when we expand the functionality of the device to support such shared memory.
Best regards,
thanks,
Takashi