Re: [PATCH v2 05/12] iio: core: Add new DMABUF interface infrastructure

Jonathan Cameron
2022-03-29T13:37:03Z

On Mon, 28 Mar 2022 19:44:19 +0100
Paul Cercueil <paul@crapouillou.net> wrote:

> Hi Jonathan,
>
> Le lun., mars 28 2022 at 18:37:01 +0100, Jonathan Cameron
> <jic23@kernel.org> a écrit :
> > On Mon, 7 Feb 2022 12:59:26 +0000
> > Paul Cercueil <paul@crapouillou.net> wrote:
> >
> >> Add the necessary infrastructure to the IIO core to support a new
> >> optional DMABUF based interface.
> >>
> >> The advantage of this new DMABUF based interface vs. the read()
> >> interface, is that it avoids an extra copy of the data between the
> >> kernel and userspace. This is particularly userful for high-speed
> >
> > useful
> >
> >> devices which produce several megabytes or even gigabytes of data
> >> per
> >> second.
> >>
> >> The data in this new DMABUF interface is managed at the granularity
> >> of
> >> DMABUF objects. Reducing the granularity from byte level to block
> >> level
> >> is done to reduce the userspace-kernelspace synchronization overhead
> >> since performing syscalls for each byte at a few Mbps is just not
> >> feasible.
> >>
> >> This of course leads to a slightly increased latency. For this
> >> reason an
> >> application can choose the size of the DMABUFs as well as how many
> >> it
> >> allocates. E.g. two DMABUFs would be a traditional double buffering
> >> scheme. But using a higher number might be necessary to avoid
> >> underflow/overflow situations in the presence of scheduling
> >> latencies.
> >>
> >> As part of the interface, 2 new IOCTLs have been added:
> >>
> >> IIO_BUFFER_DMABUF_ALLOC_IOCTL(struct iio_dmabuf_alloc_req *):
> >> Each call will allocate a new DMABUF object. The return value (if
> >> not
> >> a negative errno value as error) will be the file descriptor of
> >> the new
> >> DMABUF.
> >>
> >> IIO_BUFFER_DMABUF_ENQUEUE_IOCTL(struct iio_dmabuf *):
> >> Place the DMABUF object into the queue pending for hardware
> >> process.
> >>
> >> These two IOCTLs have to be performed on the IIO buffer's file
> >> descriptor, obtained using the IIO_BUFFER_GET_FD_IOCTL() ioctl.
> >
> > Just to check, do they work on the old deprecated chardev route?
> > Normally
> > we can directly access the first buffer without the ioctl.
>
> They do not. I think it's fine this way, since as you said, the old
> chardev route is deprecated. But I can add support for it with enough
> peer pressure.
Agreed. Definitely fine to not support the 'old way'.

J
>
> >>
> >> To access the data stored in a block by userspace the block must be
> >> mapped to the process's memory. This is done by calling mmap() on
> >> the
> >> DMABUF's file descriptor.
> >>
> >> Before accessing the data through the map, you must use the
> >> DMA_BUF_IOCTL_SYNC(struct dma_buf_sync *) ioctl, with the
> >> DMA_BUF_SYNC_START flag, to make sure that the data is available.
> >> This call may block until the hardware is done with this block. Once
> >> you are done reading or writing the data, you must use this ioctl
> >> again
> >> with the DMA_BUF_SYNC_END flag, before enqueueing the DMABUF to the
> >> kernel's queue.
> >>
> >> If you need to know when the hardware is done with a DMABUF, you can
> >> poll its file descriptor for the EPOLLOUT event.
> >>
> >> Finally, to destroy a DMABUF object, simply call close() on its file
> >> descriptor.
> >>
> >> A typical workflow for the new interface is:
> >>
> >> for block in blocks:
> >> DMABUF_ALLOC block
> >> mmap block
> >>
> >> enable buffer
> >>
> >> while !done
> >> for block in blocks:
> >> DMABUF_ENQUEUE block
> >>
> >> DMABUF_SYNC_START block
> >> process data
> >> DMABUF_SYNC_END block
> >>
> >> disable buffer
> >>
> >> for block in blocks:
> >> close block
> >
> > Given my very limited knowledge of dma-buf, I'll leave commenting
> > on the flow to others who know if this looks 'standards' or not ;)
> >
> > Code looks sane to me..
>
> Thanks.
>
> Cheers,
> -Paul
>
> >>
> >> v2: Only allow the new IOCTLs on the buffer FD created with
> >> IIO_BUFFER_GET_FD_IOCTL().
> >>
> >> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> >> ---
> >> drivers/iio/industrialio-buffer.c | 55
> >> +++++++++++++++++++++++++++++++
> >> include/linux/iio/buffer_impl.h | 8 +++++
> >> include/uapi/linux/iio/buffer.h | 29 ++++++++++++++++
> >> 3 files changed, 92 insertions(+)
> >>
> >> diff --git a/drivers/iio/industrialio-buffer.c
> >> b/drivers/iio/industrialio-buffer.c
> >> index 94eb9f6cf128..72f333a519bc 100644
> >> --- a/drivers/iio/industrialio-buffer.c
> >> +++ b/drivers/iio/industrialio-buffer.c
> >> @@ -17,6 +17,7 @@
> >> #include <linux/fs.h>
> >> #include <linux/cdev.h>
> >> #include <linux/slab.h>
> >> +#include <linux/mm.h>
> >> #include <linux/poll.h>
> >> #include <linux/sched/signal.h>
> >>
> >> @@ -1520,11 +1521,65 @@ static int iio_buffer_chrdev_release(struct
> >> inode *inode, struct file *filep)
> >> return 0;
> >> }
> >>
> >> +static int iio_buffer_enqueue_dmabuf(struct iio_buffer *buffer,
> >> + struct iio_dmabuf __user *user_buf)
> >> +{
> >> + struct iio_dmabuf dmabuf;
> >> +
> >> + if (!buffer->access->enqueue_dmabuf)
> >> + return -EPERM;
> >> +
> >> + if (copy_from_user(&dmabuf, user_buf, sizeof(dmabuf)))
> >> + return -EFAULT;
> >> +
> >> + if (dmabuf.flags & ~IIO_BUFFER_DMABUF_SUPPORTED_FLAGS)
> >> + return -EINVAL;
> >> +
> >> + return buffer->access->enqueue_dmabuf(buffer, &dmabuf);
> >> +}
> >> +
> >> +static int iio_buffer_alloc_dmabuf(struct iio_buffer *buffer,
> >> + struct iio_dmabuf_alloc_req __user *user_req)
> >> +{
> >> + struct iio_dmabuf_alloc_req req;
> >> +
> >> + if (!buffer->access->alloc_dmabuf)
> >> + return -EPERM;
> >> +
> >> + if (copy_from_user(&req, user_req, sizeof(req)))
> >> + return -EFAULT;
> >> +
> >> + if (req.resv)
> >> + return -EINVAL;
> >> +
> >> + return buffer->access->alloc_dmabuf(buffer, &req);
> >> +}
> >> +
> >> +static long iio_buffer_chrdev_ioctl(struct file *filp,
> >> + unsigned int cmd, unsigned long arg)
> >> +{
> >> + struct iio_dev_buffer_pair *ib = filp->private_data;
> >> + struct iio_buffer *buffer = ib->buffer;
> >> + void __user *_arg = (void __user *)arg;
> >> +
> >> + switch (cmd) {
> >> + case IIO_BUFFER_DMABUF_ALLOC_IOCTL:
> >> + return iio_buffer_alloc_dmabuf(buffer, _arg);
> >> + case IIO_BUFFER_DMABUF_ENQUEUE_IOCTL:
> >> + /* TODO: support non-blocking enqueue operation */
> >> + return iio_buffer_enqueue_dmabuf(buffer, _arg);
> >> + default:
> >> + return IIO_IOCTL_UNHANDLED;
> >> + }
> >> +}
> >> +
> >> static const struct file_operations iio_buffer_chrdev_fileops = {
> >> .owner = THIS_MODULE,
> >> .llseek = noop_llseek,
> >> .read = iio_buffer_read,
> >> .write = iio_buffer_write,
> >> + .unlocked_ioctl = iio_buffer_chrdev_ioctl,
> >> + .compat_ioctl = compat_ptr_ioctl,
> >> .poll = iio_buffer_poll,
> >> .release = iio_buffer_chrdev_release,
> >> };
> >> diff --git a/include/linux/iio/buffer_impl.h
> >> b/include/linux/iio/buffer_impl.h
> >> index e2ca8ea23e19..728541bc2c63 100644
> >> --- a/include/linux/iio/buffer_impl.h
> >> +++ b/include/linux/iio/buffer_impl.h
> >> @@ -39,6 +39,9 @@ struct iio_buffer;
> >> * device stops sampling. Calles are balanced
> >> with @enable.
> >> * @release: called when the last reference to the buffer is
> >> dropped,
> >> * should free all resources allocated by the buffer.
> >> + * @alloc_dmabuf: called from userspace via ioctl to allocate one
> >> DMABUF.
> >> + * @enqueue_dmabuf: called from userspace via ioctl to queue this
> >> DMABUF
> >> + * object to this buffer. Requires a valid DMABUF fd.
> >> * @modes: Supported operating modes by this buffer type
> >> * @flags: A bitmask combination of INDIO_BUFFER_FLAG_*
> >> *
> >> @@ -68,6 +71,11 @@ struct iio_buffer_access_funcs {
> >>
> >> void (*release)(struct iio_buffer *buffer);
> >>
> >> + int (*alloc_dmabuf)(struct iio_buffer *buffer,
> >> + struct iio_dmabuf_alloc_req *req);
> >> + int (*enqueue_dmabuf)(struct iio_buffer *buffer,
> >> + struct iio_dmabuf *block);
> >> +
> >> unsigned int modes;
> >> unsigned int flags;
> >> };
> >> diff --git a/include/uapi/linux/iio/buffer.h
> >> b/include/uapi/linux/iio/buffer.h
> >> index 13939032b3f6..e4621b926262 100644
> >> --- a/include/uapi/linux/iio/buffer.h
> >> +++ b/include/uapi/linux/iio/buffer.h
> >> @@ -5,6 +5,35 @@
> >> #ifndef _UAPI_IIO_BUFFER_H_
> >> #define _UAPI_IIO_BUFFER_H_
> >>
> >> +#include <linux/types.h>
> >> +
> >> +#define IIO_BUFFER_DMABUF_SUPPORTED_FLAGS 0x00000000
> >> +
> >> +/**
> >> + * struct iio_dmabuf_alloc_req - Descriptor for allocating IIO
> >> DMABUFs
> >> + * @size: the size of a single DMABUF
> >> + * @resv: reserved
> >> + */
> >> +struct iio_dmabuf_alloc_req {
> >> + __u64 size;
> >> + __u64 resv;
> >> +};
> >> +
> >> +/**
> >> + * struct iio_dmabuf - Descriptor for a single IIO DMABUF object
> >> + * @fd: file descriptor of the DMABUF object
> >> + * @flags: one or more IIO_BUFFER_DMABUF_* flags
> >> + * @bytes_used: number of bytes used in this DMABUF for the data
> >> transfer.
> >> + * If zero, the full buffer is used.
> >> + */
> >> +struct iio_dmabuf {
> >> + __u32 fd;
> >> + __u32 flags;
> >> + __u64 bytes_used;
> >> +};
> >> +
> >> #define IIO_BUFFER_GET_FD_IOCTL _IOWR('i', 0x91, int)
> >> +#define IIO_BUFFER_DMABUF_ALLOC_IOCTL _IOW('i', 0x92, struct
> >> iio_dmabuf_alloc_req)
> >> +#define IIO_BUFFER_DMABUF_ENQUEUE_IOCTL _IOW('i', 0x93, struct
> >> iio_dmabuf)
> >>
> >> #endif /* _UAPI_IIO_BUFFER_H_ */
> >
>
>