Re: [PATCH v4 6/8] media: virtio: Add virtio_media_driver

[Mauro Carvalho Chehab]

On Thu, 25 Jun 2026 16:18:48 -0400
Brian Daniels <briandaniels@xxxxxxxxxx> wrote:

> > > From: Alexandre Courbot <gnurou@xxxxxxxxx>
> > > 
> > > virtio_media_driver.c provides the expected driver hooks, and support
> > > for mmapping and polling.
> > > 
> > > Signed-off-by: Alexandre Courbot <gnurou@xxxxxxxxx>
> > > Co-developed-by: Brian Daniels <briandaniels@xxxxxxxxxx>
> > > Signed-off-by: Brian Daniels <briandaniels@xxxxxxxxxx>
> > > ---
> > >  drivers/media/virtio/virtio_media_driver.c | 959 +++++++++++++++++++++
> > >  1 file changed, 959 insertions(+)
> > >  create mode 100644 drivers/media/virtio/virtio_media_driver.c
> > > 
> > > diff --git a/drivers/media/virtio/virtio_media_driver.c b/drivers/media/virtio/virtio_media_driver.c
> > > new file mode 100644
> > > index 000000000..d6363c673
> > > --- /dev/null
> > > +++ b/drivers/media/virtio/virtio_media_driver.c
> > > @@ -0,0 +1,959 @@
> > > +// SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0+
> > > +
> > > +/*
> > > + * Virtio-media driver.
> > > + *
> > > + * Copyright (c) 2024-2025 Google LLC.
> > > + */
> > > +
> > > +#include <linux/delay.h>
> > > +#include <linux/device.h>
> > > +#include <linux/dev_printk.h>
> > > +#include <linux/mm.h>
> > > +#include <linux/mutex.h>
> > > +#include <linux/scatterlist.h>
> > > +#include <linux/types.h>
> > > +#include <linux/videodev2.h>
> > > +#include <linux/vmalloc.h>
> > > +#include <linux/wait.h>
> > > +#include <linux/workqueue.h>
> > > +#include <linux/module.h>
> > > +#include <linux/moduleparam.h>
> > > +#include <linux/virtio.h>
> > > +#include <linux/virtio_config.h>
> > > +#include <linux/virtio_ids.h>
> > > +
> > > +#include <media/frame_vector.h>
> > > +#include <media/v4l2-dev.h>
> > > +#include <media/v4l2-event.h>
> > > +#include <media/videobuf2-memops.h>
> > > +#include <media/v4l2-device.h>
> > > +#include <media/v4l2-ioctl.h>
> > > +
> > > +#include "protocol.h"
> > > +#include "session.h"
> > > +#include "virtio_media.h"
> > > +
> > > +#define VIRTIO_MEDIA_NUM_EVENT_BUFS 16
> > > +
> > > +/* ID of the SHM region into which MMAP buffer will be mapped. */
> > > +#define VIRTIO_MEDIA_SHM_MMAP 0
> > > +
> > > +/*
> > > + * Name of the driver to expose to user-space.
> > > + *
> > > + * This is configurable because v4l2-compliance has workarounds specific to
> > > + * some drivers. When proxying these directly from the host, this allows it to
> > > + * apply them as needed.
> > > + */
> > > +char *virtio_media_driver_name;
> > > +module_param_named(driver_name, virtio_media_driver_name, charp, 0660);  
> > 
> > 
> > Um. What? Not how it should be handled.  
> 
> I can remove this module param. I didn't end up using this when compliance testing.
> Instead, I patched v4l-utils:
> https://lore.kernel.org/all/20260528163448.4031965-1-briandaniels@xxxxxxxxxx/
> 
> Let me know if you think the v4l-utils patch is a good approach, otherwise let
> me know how you'd prefer to address the v4l2-compliance driver-specific workounds
> when they're being proxied with virtio-media.

This kind of discussion should happen on a separate PR for v4l2-compliance,
c/c to the proper developers and maintainers of it.

> 
> > > +
> > > +/*
> > > + * Whether USERPTR buffers are allowed.
> > > + *
> > > + * This is disabled by default as USERPTR buffers are dangerous, but the option
> > > + * is left to enable them if desired.
> > > + */
> > > +bool virtio_media_allow_userptr;
> > > +module_param_named(allow_userptr, virtio_media_allow_userptr, bool, 0660);  
> > 
> > 
> > is this kind of thing common?  

There is one old media device that has it (saa7134).

> 
> To be honest, I don't really know. I'm also not that familiar with the USERPTR
> issues. I see a few references online about their use being discouraged due to
> possible race conditions, perhaps that was the original motivation for this
> parameter (I'm not the original author of this driver).
> 
> I'm open to alternatives, feel free to let me know if you have a preference.

We tend to not implement USERPTR on newer drivers. I suggest you
to place the logic with regards to V4L2_MEMORY_USERPTR on a separate
patch for further discussions.

> 
> > > +
> > > +/**
> > > + * virtio_media_session_alloc - Allocate a new session.
> > > + * @vv: virtio-media device the session belongs to.
> > > + * @id: ID of the session.
> > > + * @nonblocking_dequeue: whether dequeuing of buffers should be blocking or
> > > + * not.
> > > + *
> > > + * The ``id`` and ``list`` fields must still be set by the caller.  
> > 
> > still in what sense?  
> 
> Based on the code below, I'm not so sure that the caller is responsible for
> setting these values. They seem to be initialized in the function.
> 
> Perhaps Alexandre Courbot (the original author) would know more. Unless he
> says otherwise though I'm inclined to remove this comment.
> 
> > > + */
> > > +static struct virtio_media_session *
> > > +virtio_media_session_alloc(struct virtio_media *vv, u32 id,
> > > +			   struct file *file)
> > > +{
> > > +	struct virtio_media_session *session;
> > > +	int i;
> > > +	int ret;
> > > +
> > > +	session = kzalloc_obj(*session, GFP_KERNEL);
> > > +	if (!session)
> > > +		goto err_session;
> > > +
> > > +	session->shadow_buf = kzalloc(VIRTIO_SHADOW_BUF_SIZE, GFP_KERNEL);
> > > +	if (!session->shadow_buf)
> > > +		goto err_shadow_buf;
> > > +
> > > +	ret = sg_alloc_table(&session->command_sgs, DESC_CHAIN_MAX_LEN,
> > > +			     GFP_KERNEL);
> > > +	if (ret)
> > > +		goto err_payload_sgs;
> > > +
> > > +	session->id = id;
> > > +	session->nonblocking_dequeue = file->f_flags & O_NONBLOCK;
> > > +
> > > +	INIT_LIST_HEAD(&session->list);
> > > +	v4l2_fh_init(&session->fh, &vv->video_dev);
> > > +	virtio_media_session_fh_add(session, file);
> > > +
> > > +	for (i = 0; i <= VIRTIO_MEDIA_LAST_QUEUE; i++)
> > > +		INIT_LIST_HEAD(&session->queues[i].pending_dqbufs);
> > > +	mutex_init(&session->queues_lock);
> > > +
> > > +	init_waitqueue_head(&session->dqbuf_wait);
> > > +
> > > +	mutex_lock(&vv->sessions_lock);
> > > +	list_add_tail(&session->list, &vv->sessions);
> > > +	mutex_unlock(&vv->sessions_lock);
> > > +
> > > +	return session;
> > > +
> > > +err_payload_sgs:
> > > +	kfree(session->shadow_buf);
> > > +err_shadow_buf:
> > > +	kfree(session);
> > > +err_session:
> > > +	return ERR_PTR(-ENOMEM);
> > > +}
> > > +
> > > +/**
> > > + * virtio_media_session_free - Free all resources of a session.
> > > + * @vv: virtio-media device the session belongs to.
> > > + * @session: session to destroy.
> > > + *
> > > + * All the resources of @sesssion, as well as the backing memory of @session
> > > + * itself, are freed.  
> > 
> > why @ here and `` above? And typo in the name.  
> 
> The `@` here was an attempt to follow the guide here for referencing function
> parameters:
> https://docs.kernel.org/doc-guide/kernel-doc.html#highlights-and-cross-references

Yes. This is part of Linux Kernel kernel-doc markup: when referring to
struct fields, you should use @field (or ``field`` if one wants to place an
asterisk on it, like ``*field``).

> 
> That being said, I don't believe this file is 100% consistent with that. I will
> spend some time cleaning up the comments throughout this patch set to get them
> consistent for v5. Thanks!

Please use it on a consistent way along the driver.


Thanks,
Mauro