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Subject: Re: [PATCH v2 5/9] ALSA: virtio: handling control and I/O messages for the PCM device
On Sun, 24 Jan 2021, Anton Yakovlev wrote:
The driver implements a message-based transport for I/O substream
operations. Before the start of the substream, the hardware buffer is
sliced into I/O messages, the number of which is equal to the current
number of periods. The size of each message is equal to the current
size of one period.
I/O messages are organized in an ordered queue. The completion of the
I/O message indicates an elapsed period (the only exception is the end
of the stream for the capture substream). Upon completion, the message
is automatically re-added to the end of the queue.
Signed-off-by: Anton Yakovlev <anton.yakovlev@opensynergy.com>
---
sound/virtio/Makefile | 3 +-
sound/virtio/virtio_card.c | 10 ++
sound/virtio/virtio_card.h | 9 +
sound/virtio/virtio_pcm.c | 3 +
sound/virtio/virtio_pcm.h | 31 ++++
sound/virtio/virtio_pcm_msg.c | 325 ++++++++++++++++++++++++++++++++++
6 files changed, 380 insertions(+), 1 deletion(-)
create mode 100644 sound/virtio/virtio_pcm_msg.c
diff --git a/sound/virtio/Makefile b/sound/virtio/Makefile
index 69162a545a41..626af3cc3ed7 100644
--- a/sound/virtio/Makefile
+++ b/sound/virtio/Makefile
@@ -5,5 +5,6 @@ obj-$(CONFIG_SND_VIRTIO) += virtio_snd.o
virtio_snd-objs := \
virtio_card.o \
virtio_ctl_msg.o \
- virtio_pcm.o
+ virtio_pcm.o \
+ virtio_pcm_msg.o
diff --git a/sound/virtio/virtio_card.c b/sound/virtio/virtio_card.c
index 39fe13b43dd1..11d025ee77c2 100644
--- a/sound/virtio/virtio_card.c
+++ b/sound/virtio/virtio_card.c
@@ -143,6 +143,12 @@ static int virtsnd_find_vqs(struct virtio_snd *snd)
callbacks[VIRTIO_SND_VQ_CONTROL] = virtsnd_ctl_notify_cb;
callbacks[VIRTIO_SND_VQ_EVENT] = virtsnd_event_notify_cb;
+ virtio_cread(vdev, struct virtio_snd_config, streams, &n);
+ if (n) {
+ callbacks[VIRTIO_SND_VQ_TX] = virtsnd_pcm_tx_notify_cb;
+ callbacks[VIRTIO_SND_VQ_RX] = virtsnd_pcm_rx_notify_cb;
+ }
+
rc = virtio_find_vqs(vdev, VIRTIO_SND_VQ_MAX, vqs, callbacks, names,
NULL);
if (rc) {
@@ -177,6 +183,10 @@ static int virtsnd_find_vqs(struct virtio_snd *snd)
* virtsnd_enable_event_vq() - Enable the event virtqueue.
* @snd: VirtIO sound device.
*
+ * The tx queue is enabled only if the device supports playback stream(s).
+ *
+ * The rx queue is enabled only if the device supports capture stream(s).
+ *
* Context: Any context.
*/
static void virtsnd_enable_event_vq(struct virtio_snd *snd)
diff --git a/sound/virtio/virtio_card.h b/sound/virtio/virtio_card.h
index be6651a6aaf8..b11c09984882 100644
--- a/sound/virtio/virtio_card.h
+++ b/sound/virtio/virtio_card.h
@@ -89,4 +89,13 @@ virtsnd_rx_queue(struct virtio_snd *snd)
return &snd->queues[VIRTIO_SND_VQ_RX];
}
+static inline struct virtio_snd_queue *
+virtsnd_pcm_queue(struct virtio_pcm_substream *substream)
+{
+ if (substream->direction == SNDRV_PCM_STREAM_PLAYBACK)
+ return virtsnd_tx_queue(substream->snd);
+ else
+ return virtsnd_rx_queue(substream->snd);
+}
+
#endif /* VIRTIO_SND_CARD_H */
diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c
index 036990b7b78a..1ab50dcc88c8 100644
--- a/sound/virtio/virtio_pcm.c
+++ b/sound/virtio/virtio_pcm.c
@@ -376,6 +376,7 @@ int virtsnd_pcm_parse_cfg(struct virtio_snd *snd)
substream->snd = snd;
substream->sid = i;
+ init_waitqueue_head(&substream->msg_empty);
rc = virtsnd_pcm_build_hw(substream, &info[i]);
if (rc)
@@ -530,6 +531,8 @@ void virtsnd_pcm_event(struct virtio_snd *snd, struct virtio_snd_event *event)
break;
}
case VIRTIO_SND_EVT_PCM_XRUN: {
+ if (atomic_read(&substream->xfer_enabled))
Why does .xfer_enabled have to be atomic? It only takes two values - 0 and 1, I don't see any incrementing, or test-and-set type operations or anything similar. Also I don't see .xfer_enabled being set to 1 anywhere in this patch, presumably that happens in one of later patches.
+ atomic_set(&substream->xfer_xrun, 1);
Ditto.
break;
}
}
diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
index 73fb4d9dc524..d011b7e1d18d 100644
--- a/sound/virtio/virtio_pcm.h
+++ b/sound/virtio/virtio_pcm.h
@@ -24,6 +24,7 @@
#include <sound/pcm.h>
struct virtio_pcm;
+struct virtio_pcm_msg;
/**
* struct virtio_pcm_substream - VirtIO PCM substream.
@@ -34,6 +35,16 @@ struct virtio_pcm;
* @features: Stream VirtIO feature bit map (1 << VIRTIO_SND_PCM_F_XXX).
* @substream: Kernel ALSA substream.
* @hw: Kernel ALSA substream hardware descriptor.
+ * @frame_bytes: Current frame size in bytes.
+ * @period_size: Current period size in frames.
+ * @buffer_size: Current buffer size in frames.
+ * @hw_ptr: Substream hardware pointer value in frames [0 ... buffer_size).
+ * @xfer_enabled: Data transfer state (0 - off, 1 - on).
+ * @xfer_xrun: Data underflow/overflow state (0 - no xrun, 1 - xrun).
+ * @msgs: I/O messages.
+ * @msg_last_enqueued: Index of the last I/O message added to the virtqueue.
+ * @msg_count: Number of pending I/O messages in the virtqueue.
+ * @msg_empty: Notify when msg_count is zero.
*/
struct virtio_pcm_substream {
struct virtio_snd *snd;
@@ -43,6 +54,16 @@ struct virtio_pcm_substream {
u32 features;
struct snd_pcm_substream *substream;
struct snd_pcm_hardware hw;
+ unsigned int frame_bytes;
+ snd_pcm_uframes_t period_size;
+ snd_pcm_uframes_t buffer_size;
+ atomic_t hw_ptr;
+ atomic_t xfer_enabled;
+ atomic_t xfer_xrun;
+ struct virtio_pcm_msg *msgs;
+ int msg_last_enqueued;
+ atomic_t msg_count;
+ wait_queue_head_t msg_empty;
};
/**
@@ -86,4 +107,14 @@ struct virtio_pcm *virtsnd_pcm_find(struct virtio_snd *snd, unsigned int nid);
struct virtio_pcm *virtsnd_pcm_find_or_create(struct virtio_snd *snd,
unsigned int nid);
+struct virtio_snd_msg *
+virtsnd_pcm_ctl_msg_alloc(struct virtio_pcm_substream *substream,
+ unsigned int command, gfp_t gfp);
+
+int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *substream,
+ unsigned int nmsg, u8 *dma_area,
+ unsigned int period_bytes);
+
+int virtsnd_pcm_msg_send(struct virtio_pcm_substream *substream);
+
#endif /* VIRTIO_SND_PCM_H */
diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c
new file mode 100644
index 000000000000..d524e7cbe43f
--- /dev/null
+++ b/sound/virtio/virtio_pcm_msg.c
@@ -0,0 +1,325 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Sound card driver for virtio
+ * Copyright (C) 2020 OpenSynergy GmbH
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+#include <sound/pcm_params.h>
+
+#include "virtio_card.h"
+
+/**
+ * enum pcm_msg_sg_index - Scatter-gather element indexes for an I/O message.
+ * @PCM_MSG_SG_XFER: Element containing a virtio_snd_pcm_xfer structure.
+ * @PCM_MSG_SG_DATA: Element containing a data buffer.
+ * @PCM_MSG_SG_STATUS: Element containing a virtio_snd_pcm_status structure.
+ * @PCM_MSG_SG_MAX: The maximum number of elements in the scatter-gather table.
+ *
+ * These values are used as the index of the payload scatter-gather table.
+ */
+enum pcm_msg_sg_index {
+ PCM_MSG_SG_XFER = 0,
+ PCM_MSG_SG_DATA,
+ PCM_MSG_SG_STATUS,
+ PCM_MSG_SG_MAX
+};
If I understand correctly, messages are sent to the back-end driver in this specific order, so this is a part of the ABI, isn't it? Is it also a part of the spec? If so this should be defined in your ABI header?
+
+/**
+ * struct virtio_pcm_msg - VirtIO I/O message.
+ * @substream: VirtIO PCM substream.
+ * @xfer: Request header payload.
+ * @status: Response header payload.
+ * @sgs: Payload scatter-gather table.
+ */
+struct virtio_pcm_msg {
+ struct virtio_pcm_substream *substream;
+ struct virtio_snd_pcm_xfer xfer;
+ struct virtio_snd_pcm_status status;
+ struct scatterlist sgs[PCM_MSG_SG_MAX];
+};
+
+/**
+ * virtsnd_pcm_msg_alloc() - Allocate I/O messages.
+ * @substream: VirtIO PCM substream.
+ * @nmsg: Number of messages (equal to the number of periods).
+ * @dma_area: Pointer to used audio buffer.
+ * @period_bytes: Period (message payload) size.
+ *
+ * The function slices the buffer into nmsg parts (each with the size of
+ * period_bytes), and creates nmsg corresponding I/O messages.
+ *
+ * Context: Any context that permits to sleep.
+ * Return: 0 on success, -ENOMEM on failure.
+ */
+int virtsnd_pcm_msg_alloc(struct virtio_pcm_substream *substream,
+ unsigned int nmsg, u8 *dma_area,
+ unsigned int period_bytes)
+{
+ struct virtio_device *vdev = substream->snd->vdev;
+ unsigned int i;
+
+ if (substream->msgs)
+ devm_kfree(&vdev->dev, substream->msgs);
+
+ substream->msgs = devm_kcalloc(&vdev->dev, nmsg,
+ sizeof(*substream->msgs), GFP_KERNEL);
+ if (!substream->msgs)
+ return -ENOMEM;
+
+ for (i = 0; i < nmsg; ++i) {
+ struct virtio_pcm_msg *msg = &substream->msgs[i];
+
+ msg->substream = substream;
+
+ sg_init_table(msg->sgs, PCM_MSG_SG_MAX);
Why do you need to initialise a table of 3 meddages if you then initialise each of them separately immediately below?
+ sg_init_one(&msg->sgs[PCM_MSG_SG_XFER], &msg->xfer,
+ sizeof(msg->xfer));
+ sg_init_one(&msg->sgs[PCM_MSG_SG_DATA],
+ dma_area + period_bytes * i, period_bytes);
+ sg_init_one(&msg->sgs[PCM_MSG_SG_STATUS], &msg->status,
+ sizeof(msg->status));
+ }
+
+ return 0;
+}
+
+/**
+ * virtsnd_pcm_msg_send() - Send asynchronous I/O messages.
+ * @substream: VirtIO PCM substream.
+ *
+ * All messages are organized in an ordered circular list. Each time the
+ * function is called, all currently non-enqueued messages are added to the
+ * virtqueue. For this, the function keeps track of two values:
+ *
+ * msg_last_enqueued = index of the last enqueued message,
+ * msg_count = # of pending messages in the virtqueue.
+ *
+ * Context: Any context.
+ * Return: 0 on success, -EIO on failure.
+ */
+int virtsnd_pcm_msg_send(struct virtio_pcm_substream *substream)
+{
+ struct snd_pcm_runtime *runtime = substream->substream->runtime;
+ struct virtio_snd *snd = substream->snd;
+ struct virtio_device *vdev = snd->vdev;
+ struct virtqueue *vqueue = virtsnd_pcm_queue(substream)->vqueue;
+ int i;
+ int n;
+ bool notify = false;
+
+ if (!vqueue)
+ return -EIO;
Is this actually possible? That would mean a data corruption or a bug in the driver, right? In either case it can be NULL or 1 or any other invalid value, so checking for NULL doesn't seem to help a lot?
+
+ i = (substream->msg_last_enqueued + 1) % runtime->periods;
+ n = runtime->periods - atomic_read(&substream->msg_count);
+
+ for (; n; --n, i = (i + 1) % runtime->periods) {
+ struct virtio_pcm_msg *msg = &substream->msgs[i];
+ struct scatterlist *psgs[PCM_MSG_SG_MAX] = {
+ [PCM_MSG_SG_XFER] = &msg->sgs[PCM_MSG_SG_XFER],
+ [PCM_MSG_SG_DATA] = &msg->sgs[PCM_MSG_SG_DATA],
+ [PCM_MSG_SG_STATUS] = &msg->sgs[PCM_MSG_SG_STATUS]
+ };
+ int rc;
+
+ msg->xfer.stream_id = cpu_to_virtio32(vdev, substream->sid);
+ memset(&msg->status, 0, sizeof(msg->status));
+
+ atomic_inc(&substream->msg_count);
.msg_count is also accessed in virtsnd_pcm_msg_complete() which is why presumably you use atomic access. But here you already increment the count before you even begin adding the message to the virtqueue. So if virtsnd_pcm_msg_complete() preempts you here the .msg_count will be inconsistent? Possibly you need to protect both operations together: incrementing the counter and adding messages to queues.
+
+ if (substream->direction == SNDRV_PCM_STREAM_PLAYBACK)
+ rc = virtqueue_add_sgs(vqueue, psgs, 2, 1, msg,
+ GFP_ATOMIC);
+ else
+ rc = virtqueue_add_sgs(vqueue, psgs, 1, 2, msg,
+ GFP_ATOMIC);
+
+ if (rc) {
+ atomic_dec(&substream->msg_count);
+ return -EIO;
+ }
+
+ substream->msg_last_enqueued = i;
+ }
+
+ if (!(substream->features & (1U << VIRTIO_SND_PCM_F_MSG_POLLING)))
+ notify = virtqueue_kick_prepare(vqueue);
+
+ if (notify)
+ if (!virtqueue_notify(vqueue))
+ return -EIO;
+
+ return 0;
+}
+
+/**
+ * virtsnd_pcm_msg_complete() - Complete an I/O message.
+ * @msg: I/O message.
+ * @size: Number of bytes written.
+ *
+ * Completion of the message means the elapsed period.
+ *
+ * The interrupt handler modifies three fields of the substream structure
+ * (hw_ptr, xfer_xrun, msg_count) that are used in operator functions. These
+ * values are atomic to avoid frequent interlocks with the interrupt handler.
+ * This becomes especially important in the case of multiple running substreams
+ * that share both the virtqueue and interrupt handler.
+ *
+ * Context: Interrupt context.
+ */
+static void virtsnd_pcm_msg_complete(struct virtio_pcm_msg *msg, size_t size)
+{
+ struct virtio_pcm_substream *substream = msg->substream;
+ snd_pcm_uframes_t hw_ptr;
+ unsigned int msg_count;
+
+ /*
+ * hw_ptr always indicates the buffer position of the first I/O message
+ * in the virtqueue. Therefore, on each completion of an I/O message,
+ * the hw_ptr value is unconditionally advanced.
+ */
+ hw_ptr = (snd_pcm_uframes_t)atomic_read(&substream->hw_ptr);
Also unclear why this has to be atomic, especially taking into account that it's only accessed in "interrupt context."
+
+ /*
+ * If the capture substream returned an incorrect status, then just
+ * increase the hw_ptr by the period size.
+ */
+ if (substream->direction == SNDRV_PCM_STREAM_PLAYBACK ||
+ size <= sizeof(msg->status)) {
+ hw_ptr += substream->period_size;
+ } else {
+ size -= sizeof(msg->status);
+ hw_ptr += size / substream->frame_bytes;
+ }
+
+ atomic_set(&substream->hw_ptr, (u32)(hw_ptr % substream->buffer_size));
+ atomic_set(&substream->xfer_xrun, 0);
+
+ msg_count = atomic_dec_return(&substream->msg_count);
+
+ if (atomic_read(&substream->xfer_enabled)) {
+ struct snd_pcm_runtime *runtime = substream->substream->runtime;
+
+ runtime->delay =
+ bytes_to_frames(runtime,
+ le32_to_cpu(msg->status.latency_bytes));
+
+ snd_pcm_period_elapsed(substream->substream);
+
+ virtsnd_pcm_msg_send(substream);
+ } else if (!msg_count) {
+ wake_up_all(&substream->msg_empty);
+ }
+}
Thanks Guennadi
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