Re: [PATCH] dma-buf: Implement simple read/write vfs ops

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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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