Re: [PATCH v6 03/34] iov_iter: Pass I/O direction into iov_iter_get_pages*()

[David Hildenbrand]

On 17.01.23 08:57, Christoph Hellwig wrote:
> On Mon, Jan 16, 2023 at 11:08:24PM +0000, David Howells wrote:
> > Define FOLL_SOURCE_BUF and FOLL_DEST_BUF to indicate to get_user_pages*()
> > and iov_iter_get_pages*() how the buffer is intended to be used in an I/O
> > operation.  Don't use READ and WRITE as a read I/O writes to memory and
> > vice versa - which causes confusion.
> >
> > The direction is checked against the iterator's data_source.
>
> Why can't we use the existing FOLL_WRITE?

Agreed. What I understand, David considers that confusing when 
considering the I/O side of things.

I recall that there is

DMA_BIDIRECTIONAL -> FOLL_WRITE
DMA_TO_DEVICE -> !FOLL_WRITE
DMA_FROM_DEVICE -> FOLL_WRITE

that used different defines for a different API. Such terminology would 
be easier to get ... but then, again, not sure if we really need 
acronyms here.

We're pinning pages and FOLL_WRITE defines how we (pinning the page) are 
going to access these pages: R/O or R/W. So the read vs. write is never 
from the POC of the device (DMA read will write to the page).

--
Thanks,

David / dhildenb