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

Paul Cercueil
2022-03-28T18:44:34Z

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.

>>
>> 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_ */
>