Am 16.03.20 um 09:56 schrieb Christoph Hellwig:
On Fri, Mar 13, 2020 at 09:17:42AM -0300, Jason Gunthorpe wrote:
On Fri, Mar 13, 2020 at 04:21:39AM -0700, Christoph Hellwig wrote:
On Thu, Mar 12, 2020 at 11:19:28AM -0300, Jason Gunthorpe wrote:
The non-page scatterlist is also a big concern for RDMA as we have
drivers that want the page list, so even if we did as this series
contemplates I'd have still have to split the drivers and create the
notion of a dma-only SGL.
The drivers I looked at want a list of IOVA address, aligned to the
device "page size". What other data do drivers want? Execept for the
software protocol stack drivers, which of couse need pages for the
stack futher down.
In principle it is possible to have just an aligned page list -
however the page size is variable, following certain rules, and today
the drivers still determine the correct page size largely on their
own.
Some progress was made recently to consolidate this, but more is
needed.
If the common code doesn't know the device page size in advance then
today's approach of sending largest possible dma mapped SGLs into the
device driver is best. The driver only has to do splitting.
The point was that drivers don't need pages, drivers need IOVAs. In
what form they are stuffed into the hardware is the drivers problem.
Well I would prefer if the drivers can somehow express their
requirements and get IOVA structures already in the form they need.
Converting the IOVA data from one form to another is sometimes quite
costly. Especially when it is only temporarily needed.
Regards,
Christian.
