On Fri, 20 Oct 2023 11:45:39 +0200, Matias Ezequiel Vara Larsen wrote: > > Hello Takashi, > > On Thu, Oct 19, 2023 at 09:48:03AM +0200, Takashi Iwai wrote: > > On Thu, 19 Oct 2023 03:20:19 +0200, > > Anton Yakovlev wrote: > > > > > > 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@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://ddec1-0-en-ctp.trendmicro.com:443/wis/clicktime/v1/query?url=https%3a%2f%2flore.kernel.org%2flkml%2f20231016151000.GE119987%40fedora%2ft%2f&umid=2f305b77-83e7-47b6-a461-a8ca67d0bfe2&auth=53c7c7de28b92dfd96e93d9dd61a23e634d2fbec-2d5775265e7e1741ae8eb783a3cb78ed553093c1 > > > >> > > > >> 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. > > > > Ah, then this implementation might be an overkill. You're still using > > the (intermediate) vmalloc buffer allocated via PCM managed mode, and > > the actual data is copied from/to there. So it doesn't conflict with > > the mmap operation at all. > > > > I guess that the problem you're trying to solve (the immediate data > > transfer to the queue) can be implemented rather via PCM ack callback > > instead. ALSA PCM core notifies the possible data transfer via PCM > > ack callback right after each change of appl_ptr or hw_ptr, including > > each read/write op or mmap commit. Then the driver can check the > > change of appl_ptr (or hw_ptr for capture), fetch the newly available > > data, and queue it immediately. > > > > Usually together with the use of ack callback, the driver sets > > SNDRV_PCM_INFO_SYNC_APPLPTR flag. This prevents the mmap of the PCM > > control record (not the audio data) and enforces the use of > > SNDRV_PCM_IOCTL_SYNC_PTR ioctl instead (so that the driver always gets > > the ack callback). > > > > > > Thanks for your comments. If I understand correctly, we have two > options: > 1. Use copy/fill_silence callbacks and use my own buffers thus disabling > mmap. > 2. Use mmap and the ack callback to track when appl_ptr changes thus > moving the content to the queues after it has been updated. > > Am I right? Not really. The ack callback is always effective no matter whether the stream is transferred via mmap or read/write. So, when you implement a la packet transfer via ack callback, it works for both mmap and read/write. It's used typically for drivers doing a packet-based transfer, e.g. FireWire drivers or USB-audio. OTOH, if you'd like to drop the intermediate ring buffer (enabled by snd_pcm_set_managed_buffer_all() and allocated/freed automatically at hw_params/hw_free in PCM core), you can implement PCM copy and fill_ops methods. But in this case, you can't copy to runtime->dma_area like your patch; you may need a temporary buffer of your own, or handle directly the user pointer. And, even with copy/fill_ops implementations, mmap itself isn't prohibited when the driver provides its own mmap call. This can be seen, for example, when a driver uses the iomem region as the data ring buffer. HTH, Takashi > Thanks, Matias. > > > thanks, > > > > Takashi > > > > > > > > > > > > > Best regards, > > > > > > > > > > > > > > > thanks, > > > > > > > > Takashi > > > > > > -- > > > Anton Yakovlev > > > Senior Software Engineer > > > > > > OpenSynergy GmbH > > > Rotherstr. 20, 10245 Berlin > > > > > >
