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@xxxxxxxxxxxxxxx> > Signed-off-by: Matias Ezequiel Vara Larsen <mvaralar@xxxxxxxxxx> > --- > 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://lore.kernel.org/lkml/20231016151000.GE119987@fedora/t/ > > 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. thanks, Takashi
