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

[Paul Cercueil <paul@crapouillou.net>]

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_ */
>