Hi Christoph,

On Tue, Dec 1, 2020 at 11:49 PM Christoph Hellwig [off-list ref] wrote:

On Tue, Dec 01, 2020 at 12:36:58PM +0900, Sergey Senozhatsky wrote:

quoted

Not that I have any sound experience in this area, but the helper
probably won't hurt. Do you also plan to add vmap() to that helper
or dma_alloc_noncontiguous()/sg_alloc_table_from_pages() only?

Yes, I think adding the vmap is useful, and it probably makes sense
to do that unconditionally.  I'd also include the fallback to
dma_alloc_pages when the noncontig version isn't supported in the
helper.

From the media perspective, it would be good to have the vmap
optional, similarly to the DMA_ATTR_NO_KERNEL_MAPPING attribute for
coherent allocations. Actually, in the media drivers, the need to have
a kernel mapping of the DMA buffers corresponds to a minority of the
drivers. Most of them only need to map them to the userspace.

Nevertheless, that minority actually happens to be quite widely used,
e.g. the uvcvideo driver, so we can't go to the other extreme and just
drop the vmap at all.

In any case, Sergey is going to share a preliminary patch on how the
current API would be used in the V4L2 videobuf2 framework. That should
give us more input on how such a helper could look.

Other than that, again, thanks a lot for helping with this.

Best regards,
Tomasz