[PATCH v2] dma-buf: Implement simple read/write vfs ops

[Laura Abbott]

On 04/07/2017 02:25 PM, Chris Wilson 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.
> 
> 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 at chris-wilson.co.uk>
> Cc: Laura Abbott <labbott at redhat.com>
> Cc: Sumit Semwal <sumit.semwal at linaro.org>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Sean Paul <seanpaul at chromium.org>
> ---
>  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 1ce7974a28a3..6e6e35c660c7 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -100,28 +100,104 @@ 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;
>  }
>  

Both the read/write are missing the dma_buf_begin_cpu_access calls.
When I add those these seem to work well enough for a simple
test. I can add a real Tested-by for the next version.

Thanks,
Laura

>  /**
> @@ -318,6 +394,8 @@ 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
>