On Thu, Sep 19, 2019 at 5:09 PM Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
>
> It is expected that processes will pass dma-buf fd between drivers, and
> only using the fd themselves for mmaping and synchronisation ioctls.
> However, it may be convenient for an application to read/write into the
> dma-buf, for instance a test case to check the internal
> dma_buf->ops->kmap interface. There may also be a small advantage to
> avoiding the mmap() for very simple/small operations.
>
> Note in particular, synchronisation with the device is left to the
> caller with an explicit DMA_BUF_IOCTL_SYNC, rather than done implicitly
> inside the read/write, so that the user can avoid synchronisation if
> they so choose.
>
> "It is easier to add synchronisation later, than it is to take it away."
Hm for mmap I think the explicit sync makes sense (we might even want
to do it in userspace). Not so sure it's a great idea for read/write
... I guess we'd need to see what people have/had in mind for the
userspace for this to decide.
Only other thought I have on this is that many dma-buf exporters don't
bother with the kmap/kunmap interface (probably fewer than those who
bother with kernel vmap and mmap), maybe we want at least a vmap
fallback for this. Or maybe just use that as an excuse to get more
people to wire up the kmap stuff :-)
-Daniel
> v2: Lots of little fixes, plus a real llseek() implements so that the
> first basic little test cases work!
>
> Testcase: igt/prime_rw
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Laura Abbott <labbott@xxxxxxxxxx>
> Cc: Sumit Semwal <sumit.semwal@xxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> Cc: Janusz Krzysztofik <janusz.krzysztofik@xxxxxxxxxxxxxxx>
> Tested-by: Laura Abbott <labbott@xxxxxxxxxx>
> ---
> Dusting this off again as it would be nice as a user to operate on dmabuf
> agnostic to the underyling driver. We have mmap, so naturally we would
> like to have read/write as well!
>
> ---
> drivers/dma-buf/dma-buf.c | 108 ++++++++++++++++++++++++++++++++------
> 1 file changed, 93 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 433d91d710e4..a19fc881a99c 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -135,28 +135,104 @@ static int dma_buf_mmap_internal(struct file *file, struct vm_area_struct *vma)
>
> static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
> {
> - struct dma_buf *dmabuf;
> - loff_t base;
> + struct dma_buf *dmabuf = file->private_data;
>
> if (!is_dma_buf_file(file))
> return -EBADF;
>
> - dmabuf = file->private_data;
> + return fixed_size_llseek(file, offset, whence, dmabuf->size);
> +}
>
> - /* only support discovering the end of the buffer,
> - but also allow SEEK_SET to maintain the idiomatic
> - SEEK_END(0), SEEK_CUR(0) pattern */
> - if (whence == SEEK_END)
> - base = dmabuf->size;
> - else if (whence == SEEK_SET)
> - base = 0;
> - else
> - return -EINVAL;
> +static ssize_t dma_buf_read(struct file *file,
> + char __user *ubuf, size_t remain,
> + loff_t *offset)
> +{
> + struct dma_buf *dmabuf = file->private_data;
> + unsigned long idx;
> + unsigned int start;
> + size_t total;
>
> - if (offset != 0)
> - return -EINVAL;
> + if (!is_dma_buf_file(file))
> + return -EBADF;
> +
> + total = 0;
> + idx = *offset >> PAGE_SHIFT;
> + start = offset_in_page(*offset);
> + while (remain) {
> + unsigned int len = min_t(size_t, remain, PAGE_SIZE - start);
> + unsigned int copied;
> + void *vaddr;
> +
> + if (*offset >= dmabuf->size)
> + return total;
> +
> + vaddr = dma_buf_kmap(dmabuf, idx);
> + if (!vaddr)
> + return total ?: -EIO;
> +
> + copied = copy_to_user(ubuf, vaddr + start, len);
> + dma_buf_kunmap(dmabuf, idx, vaddr);
> +
> + total += copied ?: len;
> + if (copied) {
> + *offset += copied;
> + return total ?: -EFAULT;
> + }
> +
> + remain -= len;
> + *offset += len;
> + ubuf += len;
> + start = 0;
> + idx++;
> + }
> +
> + return total;
> +}
> +
> +static ssize_t dma_buf_write(struct file *file,
> + const char __user *ubuf, size_t remain,
> + loff_t *offset)
> +{
> + struct dma_buf *dmabuf = file->private_data;
> + unsigned long idx;
> + unsigned int start;
> + size_t total;
> +
> + if (!is_dma_buf_file(file))
> + return -EBADF;
> +
> + total = 0;
> + idx = *offset >> PAGE_SHIFT;
> + start = offset_in_page(*offset);
> + while (remain) {
> + unsigned int len = min_t(size_t, remain, PAGE_SIZE - start);
> + unsigned int copied;
> + void *vaddr;
> +
> + if (*offset >= dmabuf->size)
> + return total;
> +
> + vaddr = dma_buf_kmap(dmabuf, idx);
> + if (!vaddr)
> + return total ?: -EIO;
> +
> + copied = copy_from_user(vaddr + start, ubuf, len);
> + dma_buf_kunmap(dmabuf, idx, vaddr);
> +
> + total += copied ?: len;
> + if (copied) {
> + *offset += copied;
> + return total ?: -EFAULT;
> + }
> +
> + remain -= len;
> + *offset += len;
> + ubuf += len;
> + start = 0;
> + idx++;
> + }
>
> - return base + offset;
> + return total;
> }
>
> /**
> @@ -413,6 +489,8 @@ static const struct file_operations dma_buf_fops = {
> .release = dma_buf_release,
> .mmap = dma_buf_mmap_internal,
> .llseek = dma_buf_llseek,
> + .read = dma_buf_read,
> + .write = dma_buf_write,
> .poll = dma_buf_poll,
> .unlocked_ioctl = dma_buf_ioctl,
> #ifdef CONFIG_COMPAT
> --
> 2.23.0
>
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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