Re: [PATCH v4 3/4] usb: gadget: functionfs: Add DMABUF import interface

Christian König
2024-01-17T13:15:31Z

Am 17.01.24 um 13:26 schrieb Paul Cercueil:
> This patch introduces three new ioctls. They all should be called on a
> data endpoint (ie. not ep0). They are:
>
> - FUNCTIONFS_DMABUF_ATTACH, which takes the file descriptor of a DMABUF
> object to attach to the endpoint.
>
> - FUNCTIONFS_DMABUF_DETACH, which takes the file descriptor of the
> DMABUF to detach from the endpoint. Note that closing the endpoint's
> file descriptor will automatically detach all attached DMABUFs.
>
> - FUNCTIONFS_DMABUF_TRANSFER, which requests a data transfer from / to
> the given DMABUF. Its argument is a structure that packs the DMABUF's
> file descriptor, the size in bytes to transfer (which should generally
> be set to the size of the DMABUF), and a 'flags' field which is unused
> for now.
> Before this ioctl can be used, the related DMABUF must be attached
> with FUNCTIONFS_DMABUF_ATTACH.
>
> These three ioctls enable the FunctionFS code to transfer data between
> the USB stack and a DMABUF object, which can be provided by a driver
> from a completely different subsystem, in a zero-copy fashion.
>
> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

I only looked at it from the DMA-buf maintainer point of view and
especially the fence signaling (which people often get wrong) looks
correct to me.

Can't judge if the USB side will work or not, so I can only give my
Acked-by for this as well.

Regards,
Christian.

>
> ---
> v2:
> - Make ffs_dma_resv_lock() static
> - Add MODULE_IMPORT_NS(DMA_BUF);
> - The attach/detach functions are now performed without locking the
> eps_lock spinlock. The transfer function starts with the spinlock
> unlocked, then locks it before allocating and queueing the USB
> transfer.
>
> v3:
> - Inline to_ffs_dma_fence() which was called only once.
> - Simplify ffs_dma_resv_lock()
> - Add comment explaining why we unref twice in ffs_dmabuf_detach()
> - Document uapi struct usb_ffs_dmabuf_transfer_req and IOCTLs
>
> v4:
> - Protect the dmabufs list with a mutex
> - Use incremental sequence number for the dma_fences
> - Unref attachments and DMABUFs in workers
> - Remove dead code in ffs_dma_resv_lock()
> - Fix non-block actually blocking
> - Use dma_fence_begin/end_signalling()
> - Add comment about cache-management and dma_buf_unmap_attachment()
> - Make sure dma_buf_map_attachment() is called with the dma-resv locked
> ---
> drivers/usb/gadget/function/f_fs.c | 454 ++++++++++++++++++++++++++++
> include/uapi/linux/usb/functionfs.h | 41 +++
> 2 files changed, 495 insertions(+)
>
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index ed2a6d5fcef7..64dfd084c857 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -15,6 +15,9 @@
> /* #define VERBOSE_DEBUG */
>
> #include <linux/blkdev.h>
> +#include <linux/dma-buf.h>
> +#include <linux/dma-fence.h>
> +#include <linux/dma-resv.h>
> #include <linux/pagemap.h>
> #include <linux/export.h>
> #include <linux/fs_parser.h>
> @@ -43,6 +46,8 @@
>
> #define FUNCTIONFS_MAGIC 0xa647361 /* Chosen by a honest dice roll ;) */
>
> +MODULE_IMPORT_NS(DMA_BUF);
> +
> /* Reference counter handling */
> static void ffs_data_get(struct ffs_data *ffs);
> static void ffs_data_put(struct ffs_data *ffs);
> @@ -124,6 +129,23 @@ struct ffs_ep {
> u8 num;
> };
>
> +struct ffs_dmabuf_priv {
> + struct list_head entry;
> + struct kref ref;
> + struct ffs_data *ffs;
> + struct dma_buf_attachment *attach;
> + spinlock_t lock;
> + u64 context;
> +};
> +
> +struct ffs_dma_fence {
> + struct dma_fence base;
> + struct ffs_dmabuf_priv *priv;
> + struct sg_table *sgt;
> + enum dma_data_direction dir;
> + struct work_struct work;
> +};
> +
> struct ffs_epfile {
> /* Protects ep->ep and ep->req. */
> struct mutex mutex;
> @@ -197,6 +219,11 @@ struct ffs_epfile {
> unsigned char isoc; /* P: ffs->eps_lock */
>
> unsigned char _pad;
> +
> + /* Protects dmabufs */
> + struct mutex dmabufs_mutex;
> + struct list_head dmabufs; /* P: dmabufs_mutex */
> + atomic_t seqno;
> };
>
> struct ffs_buffer {
> @@ -1271,10 +1298,47 @@ static ssize_t ffs_epfile_read_iter(struct kiocb *kiocb, struct iov_iter *to)
> return res;
> }
>
> +static void ffs_dmabuf_release(struct kref *ref)
> +{
> + struct ffs_dmabuf_priv *priv = container_of(ref, struct ffs_dmabuf_priv, ref);
> + struct dma_buf_attachment *attach = priv->attach;
> + struct dma_buf *dmabuf = attach->dmabuf;
> +
> + pr_debug("FFS DMABUF release\n");
> + dma_buf_detach(attach->dmabuf, attach);
> + dma_buf_put(dmabuf);
> + kfree(priv);
> +}
> +
> +static void ffs_dmabuf_get(struct dma_buf_attachment *attach)
> +{
> + struct ffs_dmabuf_priv *priv = attach->importer_priv;
> +
> + kref_get(&priv->ref);
> +}
> +
> +static void ffs_dmabuf_put(struct dma_buf_attachment *attach)
> +{
> + struct ffs_dmabuf_priv *priv = attach->importer_priv;
> +
> + kref_put(&priv->ref, ffs_dmabuf_release);
> +}
> +
> static int
> ffs_epfile_release(struct inode *inode, struct file *file)
> {
> struct ffs_epfile *epfile = inode->i_private;
> + struct ffs_dmabuf_priv *priv, *tmp;
> +
> + mutex_lock(&epfile->dmabufs_mutex);
> +
> + /* Close all attached DMABUFs */
> + list_for_each_entry_safe(priv, tmp, &epfile->dmabufs, entry) {
> + list_del(&priv->entry);
> + ffs_dmabuf_put(priv->attach);
> + }
> +
> + mutex_unlock(&epfile->dmabufs_mutex);
>
> __ffs_epfile_read_buffer_free(epfile);
> ffs_data_closed(epfile->ffs);
> @@ -1282,6 +1346,356 @@ ffs_epfile_release(struct inode *inode, struct file *file)
> return 0;
> }
>
> +static void ffs_dmabuf_unmap_work(struct work_struct *work)
> +{
> + struct ffs_dma_fence *dma_fence =
> + container_of(work, struct ffs_dma_fence, work);
> + struct ffs_dmabuf_priv *priv = dma_fence->priv;
> + struct dma_buf_attachment *attach = priv->attach;
> + struct dma_fence *fence = &dma_fence->base;
> +
> + dma_resv_lock(attach->dmabuf->resv, NULL);
> + dma_buf_unmap_attachment(attach, dma_fence->sgt, dma_fence->dir);
> + dma_resv_unlock(attach->dmabuf->resv);
> +
> + ffs_dmabuf_put(attach);
> + dma_fence_put(fence);
> +}
> +
> +static void ffs_dmabuf_signal_done(struct ffs_dma_fence *dma_fence, int ret)
> +{
> + struct ffs_dmabuf_priv *priv = dma_fence->priv;
> + struct dma_fence *fence = &dma_fence->base;
> + bool cookie = dma_fence_begin_signalling();
> +
> + /*
> + * The dma_buf_unmap_attachment() also perform cache-management.
> + * In a perfect world this would be done before the DMA fence is
> + * signaled; however this is not yet possible as
> + * dma_buf_unmap_attachment() is not safe to use in the DMA fence's
> + * critical section.
> + */
> +
> + dma_fence_get(fence);
> + fence->error = ret;
> + dma_fence_signal(fence);
> + dma_fence_end_signalling(cookie);
> +
> + /*
> + * The fence will be unref'd in ffs_dmabuf_unmap_work.
> + * It can't be done here, as the unref functions might try to lock
> + * the resv object, which would deadlock.
> + */
> + INIT_WORK(&dma_fence->work, ffs_dmabuf_unmap_work);
> + queue_work(priv->ffs->io_completion_wq, &dma_fence->work);
> +}
> +
> +static void ffs_epfile_dmabuf_io_complete(struct usb_ep *ep,
> + struct usb_request *req)
> +{
> + pr_debug("FFS: DMABUF transfer complete, status=%d\n", req->status);
> + ffs_dmabuf_signal_done(req->context, req->status);
> + usb_ep_free_request(ep, req);
> +}
> +
> +static const char *ffs_dmabuf_get_driver_name(struct dma_fence *fence)
> +{
> + return "functionfs";
> +}
> +
> +static const char *ffs_dmabuf_get_timeline_name(struct dma_fence *fence)
> +{
> + return "";
> +}
> +
> +static void ffs_dmabuf_fence_release(struct dma_fence *fence)
> +{
> + struct ffs_dma_fence *dma_fence =
> + container_of(fence, struct ffs_dma_fence, base);
> +
> + kfree(dma_fence);
> +}
> +
> +static const struct dma_fence_ops ffs_dmabuf_fence_ops = {
> + .get_driver_name = ffs_dmabuf_get_driver_name,
> + .get_timeline_name = ffs_dmabuf_get_timeline_name,
> + .release = ffs_dmabuf_fence_release,
> +};
> +
> +static int ffs_dma_resv_lock(struct dma_buf *dmabuf, bool nonblock)
> +{
> + if (!nonblock)
> + return dma_resv_lock_interruptible(dmabuf->resv, NULL);
> +
> + if (!dma_resv_trylock(dmabuf->resv))
> + return -EBUSY;
> +
> + return 0;
> +}
> +
> +static struct dma_buf_attachment *
> +ffs_dmabuf_find_attachment(struct ffs_epfile *epfile, struct dma_buf *dmabuf)
> +{
> + struct device *dev = epfile->ffs->gadget->dev.parent;
> + struct dma_buf_attachment *attach = NULL;
> + struct ffs_dmabuf_priv *priv;
> +
> + mutex_lock(&epfile->dmabufs_mutex);
> +
> + list_for_each_entry(priv, &epfile->dmabufs, entry) {
> + if (priv->attach->dev == dev
> + && priv->attach->dmabuf == dmabuf) {
> + attach = priv->attach;
> + break;
> + }
> + }
> +
> + if (attach)
> + ffs_dmabuf_get(attach);
> +
> + mutex_unlock(&epfile->dmabufs_mutex);
> +
> + return attach ?: ERR_PTR(-EPERM);
> +}
> +
> +static int ffs_dmabuf_attach(struct file *file, int fd)
> +{
> + struct ffs_epfile *epfile = file->private_data;
> + struct usb_gadget *gadget = epfile->ffs->gadget;
> + struct dma_buf_attachment *attach;
> + struct ffs_dmabuf_priv *priv;
> + struct dma_buf *dmabuf;
> + int err;
> +
> + if (!gadget || !gadget->sg_supported)
> + return -EPERM;
> +
> + dmabuf = dma_buf_get(fd);
> + if (IS_ERR(dmabuf))
> + return PTR_ERR(dmabuf);
> +
> + attach = dma_buf_attach(dmabuf, gadget->dev.parent);
> + if (IS_ERR(attach)) {
> + err = PTR_ERR(attach);
> + goto err_dmabuf_put;
> + }
> +
> + priv = kzalloc(sizeof(*priv), GFP_KERNEL);
> + if (!priv) {
> + err = -ENOMEM;
> + goto err_dmabuf_detach;
> + }
> +
> + attach->importer_priv = priv;
> +
> + priv->ffs = epfile->ffs;
> + priv->attach = attach;
> + spin_lock_init(&priv->lock);
> + kref_init(&priv->ref);
> + priv->context = dma_fence_context_alloc(1);
> +
> + mutex_lock(&epfile->dmabufs_mutex);
> + list_add(&priv->entry, &epfile->dmabufs);
> + mutex_unlock(&epfile->dmabufs_mutex);
> +
> + return 0;
> +
> +err_dmabuf_detach:
> + dma_buf_detach(dmabuf, attach);
> +err_dmabuf_put:
> + dma_buf_put(dmabuf);
> +
> + return err;
> +}
> +
> +static int ffs_dmabuf_detach(struct file *file, int fd)
> +{
> + struct ffs_epfile *epfile = file->private_data;
> + struct device *dev = epfile->ffs->gadget->dev.parent;
> + struct ffs_dmabuf_priv *priv;
> + struct dma_buf *dmabuf;
> + int ret = -EPERM;
> +
> + dmabuf = dma_buf_get(fd);
> + if (IS_ERR(dmabuf))
> + return PTR_ERR(dmabuf);
> +
> + mutex_lock(&epfile->dmabufs_mutex);
> +
> + list_for_each_entry(priv, &epfile->dmabufs, entry) {
> + if (priv->attach->dev == dev
> + && priv->attach->dmabuf == dmabuf) {
> + list_del(&priv->entry);
> +
> + /* Unref the reference from ffs_dmabuf_attach() */
> + ffs_dmabuf_put(priv->attach);
> + ret = 0;
> + break;
> + }
> + }
> +
> + mutex_unlock(&epfile->dmabufs_mutex);
> + dma_buf_put(dmabuf);
> +
> + return ret;
> +}
> +
> +static int ffs_dmabuf_transfer(struct file *file,
> + const struct usb_ffs_dmabuf_transfer_req *req)
> +{
> + bool dma_to_ram, nonblock = file->f_flags & O_NONBLOCK;
> + struct ffs_epfile *epfile = file->private_data;
> + struct dma_buf_attachment *attach;
> + struct ffs_dmabuf_priv *priv;
> + enum dma_data_direction dir;
> + struct ffs_dma_fence *fence;
> + struct usb_request *usb_req;
> + struct sg_table *sg_table;
> + struct dma_buf *dmabuf;
> + struct ffs_ep *ep;
> + bool cookie;
> + u32 seqno;
> + int ret;
> +
> + if (req->flags & ~USB_FFS_DMABUF_TRANSFER_MASK)
> + return -EINVAL;
> +
> + dmabuf = dma_buf_get(req->fd);
> + if (IS_ERR(dmabuf))
> + return PTR_ERR(dmabuf);
> +
> + if (req->length > dmabuf->size || req->length == 0) {
> + ret = -EINVAL;
> + goto err_dmabuf_put;
> + }
> +
> + attach = ffs_dmabuf_find_attachment(epfile, dmabuf);
> + if (IS_ERR(attach)) {
> + ret = PTR_ERR(attach);
> + goto err_dmabuf_put;
> + }
> +
> + priv = attach->importer_priv;
> +
> + if (epfile->in)
> + dir = DMA_FROM_DEVICE;
> + else
> + dir = DMA_TO_DEVICE;
> +
> + ep = ffs_epfile_wait_ep(file);
> + if (IS_ERR(ep)) {
> + ret = PTR_ERR(ep);
> + goto err_attachment_put;
> + }
> +
> + ret = ffs_dma_resv_lock(dmabuf, nonblock);
> + if (ret)
> + goto err_attachment_put;
> +
> + /* Make sure we don't have writers */
> + if (!dma_resv_test_signaled(dmabuf->resv, DMA_RESV_USAGE_WRITE)) {
> + pr_debug("FFS WRITE fence is not signaled\n");
> + ret = -EBUSY;
> + goto err_resv_unlock;
> + }
> +
> + dma_to_ram = dir == DMA_FROM_DEVICE;
> +
> + /* If we're writing to the DMABUF, make sure we don't have readers */
> + if (dma_to_ram &&
> + !dma_resv_test_signaled(dmabuf->resv, DMA_RESV_USAGE_READ)) {
> + pr_debug("FFS READ fence is not signaled\n");
> + ret = -EBUSY;
> + goto err_resv_unlock;
> + }
> +
> + sg_table = dma_buf_map_attachment(attach, dir);
> + if (IS_ERR(sg_table)) {
> + ret = PTR_ERR(sg_table);
> + goto err_resv_unlock;
> + }
> +
> + ret = dma_resv_reserve_fences(dmabuf->resv, 1);
> + if (ret)
> + goto err_unmap_attachment;
> +
> + fence = kmalloc(sizeof(*fence), GFP_KERNEL);
> + if (!fence) {
> + ret = -ENOMEM;
> + goto err_resv_unlock;
> + }
> +
> + fence->sgt = sg_table;
> + fence->dir = dir;
> + fence->priv = priv;
> +
> + spin_lock_irq(&epfile->ffs->eps_lock);
> +
> + /* In the meantime, endpoint got disabled or changed. */
> + if (epfile->ep != ep) {
> + ret = -ESHUTDOWN;
> + goto err_fence_put;
> + }
> +
> + usb_req = usb_ep_alloc_request(ep->ep, GFP_ATOMIC);
> + if (!usb_req) {
> + ret = -ENOMEM;
> + goto err_fence_put;
> + }
> +
> + /*
> + * usb_ep_queue() guarantees that all transfers are processed in the
> + * order they are enqueued, so we can use a simple incrementing
> + * sequence number for the dma_fence.
> + */
> + seqno = atomic_add_return(1, &epfile->seqno);
> +
> + dma_fence_init(&fence->base, &ffs_dmabuf_fence_ops,
> + &priv->lock, priv->context, seqno);
> +
> + dma_resv_add_fence(dmabuf->resv, &fence->base,
> + dma_resv_usage_rw(dma_to_ram));
> + dma_resv_unlock(dmabuf->resv);
> +
> + /* Now that the dma_fence is in place, queue the transfer. */
> +
> + usb_req->length = req->length;
> + usb_req->buf = NULL;
> + usb_req->sg = sg_table->sgl;
> + usb_req->num_sgs = sg_nents_for_len(sg_table->sgl, req->length);
> + usb_req->sg_was_mapped = true;
> + usb_req->context = fence;
> + usb_req->complete = ffs_epfile_dmabuf_io_complete;
> +
> + cookie = dma_fence_begin_signalling();
> + ret = usb_ep_queue(ep->ep, usb_req, GFP_ATOMIC);
> + dma_fence_end_signalling(cookie);
> + if (ret) {
> + pr_warn("FFS: Failed to queue DMABUF: %d\n", ret);
> + ffs_dmabuf_signal_done(fence, ret);
> + usb_ep_free_request(ep->ep, usb_req);
> + }
> +
> + spin_unlock_irq(&epfile->ffs->eps_lock);
> + dma_buf_put(dmabuf);
> +
> + return ret;
> +
> +err_fence_put:
> + spin_unlock_irq(&epfile->ffs->eps_lock);
> + dma_fence_put(&fence->base);
> +err_unmap_attachment:
> + dma_buf_unmap_attachment(attach, sg_table, dir);
> +err_resv_unlock:
> + dma_resv_unlock(dmabuf->resv);
> +err_attachment_put:
> + ffs_dmabuf_put(attach);
> +err_dmabuf_put:
> + dma_buf_put(dmabuf);
> +
> + return ret;
> +}
> +
> static long ffs_epfile_ioctl(struct file *file, unsigned code,
> unsigned long value)
> {
> @@ -1292,6 +1706,44 @@ static long ffs_epfile_ioctl(struct file *file, unsigned code,
> if (WARN_ON(epfile->ffs->state != FFS_ACTIVE))
> return -ENODEV;
>
> + switch (code) {
> + case FUNCTIONFS_DMABUF_ATTACH:
> + {
> + int fd;
> +
> + if (copy_from_user(&fd, (void __user *)value, sizeof(fd))) {
> + ret = -EFAULT;
> + break;
> + }
> +
> + return ffs_dmabuf_attach(file, fd);
> + }
> + case FUNCTIONFS_DMABUF_DETACH:
> + {
> + int fd;
> +
> + if (copy_from_user(&fd, (void __user *)value, sizeof(fd))) {
> + ret = -EFAULT;
> + break;
> + }
> +
> + return ffs_dmabuf_detach(file, fd);
> + }
> + case FUNCTIONFS_DMABUF_TRANSFER:
> + {
> + struct usb_ffs_dmabuf_transfer_req req;
> +
> + if (copy_from_user(&req, (void __user *)value, sizeof(req))) {
> + ret = -EFAULT;
> + break;
> + }
> +
> + return ffs_dmabuf_transfer(file, &req);
> + }
> + default:
> + break;
> + }
> +
> /* Wait for endpoint to be enabled */
> ep = ffs_epfile_wait_ep(file);
> if (IS_ERR(ep))
> @@ -1869,6 +2321,8 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
> for (i = 1; i <= count; ++i, ++epfile) {
> epfile->ffs = ffs;
> mutex_init(&epfile->mutex);
> + mutex_init(&epfile->dmabufs_mutex);
> + INIT_LIST_HEAD(&epfile->dmabufs);
> if (ffs->user_flags & FUNCTIONFS_VIRTUAL_ADDR)
> sprintf(epfile->name, "ep%02x", ffs->eps_addrmap[i]);
> else
> diff --git a/include/uapi/linux/usb/functionfs.h b/include/uapi/linux/usb/functionfs.h
> index 078098e73fd3..9f88de9c3d66 100644
> --- a/include/uapi/linux/usb/functionfs.h
> +++ b/include/uapi/linux/usb/functionfs.h
> @@ -86,6 +86,22 @@ struct usb_ext_prop_desc {
> __le16 wPropertyNameLength;
> } __attribute__((packed));
>
> +/* Flags for usb_ffs_dmabuf_transfer_req->flags (none for now) */
> +#define USB_FFS_DMABUF_TRANSFER_MASK 0x0
> +
> +/**
> + * struct usb_ffs_dmabuf_transfer_req - Transfer request for a DMABUF object
> + * @fd: file descriptor of the DMABUF object
> + * @flags: one or more USB_FFS_DMABUF_TRANSFER_* flags
> + * @length: number of bytes used in this DMABUF for the data transfer.
> + * Should generally be set to the DMABUF's size.
> + */
> +struct usb_ffs_dmabuf_transfer_req {
> + int fd;
> + __u32 flags;
> + __u64 length;
> +} __attribute__((packed));
> +
> #ifndef __KERNEL__
>
> /*
> @@ -290,6 +306,31 @@ struct usb_functionfs_event {
> #define FUNCTIONFS_ENDPOINT_DESC _IOR('g', 130, \
> struct usb_endpoint_descriptor)
>
> +/*
> + * Attach the DMABUF object, identified by its file descriptor, to the
> + * data endpoint. Returns zero on success, and a negative errno value
> + * on error.
> + */
> +#define FUNCTIONFS_DMABUF_ATTACH _IOW('g', 131, int)
> +
>
> +/*
> + * Detach the given DMABUF object, identified by its file descriptor,
> + * from the data endpoint. Returns zero on success, and a negative
> + * errno value on error. Note that closing the endpoint's file
> + * descriptor will automatically detach all attached DMABUFs.
> + */
> +#define FUNCTIONFS_DMABUF_DETACH _IOW('g', 132, int)
> +
> +/*
> + * Enqueue the previously attached DMABUF to the transfer queue.
> + * The argument is a structure that packs the DMABUF's file descriptor,
> + * the size in bytes to transfer (which should generally correspond to
> + * the size of the DMABUF), and a 'flags' field which is unused
> + * for now. Returns zero on success, and a negative errno value on
> + * error.
> + */
> +#define FUNCTIONFS_DMABUF_TRANSFER _IOW('g', 133, \
> + struct usb_ffs_dmabuf_transfer_req)
>
> #endif /* _UAPI__LINUX_FUNCTIONFS_H__ */