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Date: Mon, 28 Mar 2022 19:44:19 +0100 From: Paul Cercueil <paul@...pouillou.net> To: Jonathan Cameron <jic23@...nel.org> Cc: Michael Hennerich <Michael.Hennerich@...log.com>, Lars-Peter Clausen <lars@...afoo.de>, Christian König <christian.koenig@....com>, Sumit Semwal <sumit.semwal@...aro.org>, Jonathan Corbet <corbet@....net>, Alexandru Ardelean <ardeleanalex@...il.com>, dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org Subject: Re: [PATCH v2 05/12] iio: core: Add new DMABUF interface infrastructure Hi Jonathan, Le lun., mars 28 2022 at 18:37:01 +0100, Jonathan Cameron <jic23@...nel.org> a écrit : > On Mon, 7 Feb 2022 12:59:26 +0000 > Paul Cercueil <paul@...pouillou.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@...pouillou.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_ */ >
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