On 19/10/2020 13:48, Jason Gunthorpe wrote:
On Mon, Oct 19, 2020 at 01:29:42PM +0100, Tvrtko Ursulin wrote:
On 19/10/2020 13:12, Jason Gunthorpe wrote:
On Mon, Oct 19, 2020 at 10:50:14AM +0100, Tvrtko Ursulin wrote:
overshoot the max_segment if it is not a multiple of PAGE_SIZE. Simply fix
the alignment before starting and don't expose this implementation detail
to the callers.
What does not make complete sense to me is the statement that input
alignment requirement makes it impossible to connect to DMA layer, but then
the patch goes to align behind the covers anyway.
At minimum the kerneldoc should explain that max_segment will still be
rounded down. But wouldn't it be better for the API to be more explicit and
just require aligned anyway?
Why?
The API is to not produce sge's with a length longer than max_segment,
it isn't to produce sge's of exactly max_segment.
Everything else is an internal detail
(Half-)pretending it is disconnected from PAGE_SIZE, when it really isn't,
isn't the most obvious API design in my view.
It is not information the callers need to care about
Unless they want a 1024 byte segment - then they do need to care. The
patch just changes the rules from "any non-zero aligned" to "any greater
or equal than PAGE_SIZE". I am simply asking why in more specific terms.
In other words, if you let users pass in 4097 and it just works by rounding
down to 4096, but you don't let them pass 4095, I just find this odd.
That is also an implementation detail, there is nothing preventing
smaller than page size other than complexity of implementing the
algorithm. If something ever needed it, it could be done.
But does non aligned segment size make sense? Is there any current or
future platform which will need to to warrant changing the API in this way?
My question was why not have callers pass in page aligned max segment like
today which makes it completely transparent.
Why put this confusing code in every caller? Especially for something
a driver is supposed to call. Will just make bugs
For max_segment to be aligned is a requirement today so callers are
ready. I still don't understand why change that, what will it help with?
Is it just about the desire to be able to pass in UINT_MAX?
Regards,
Tvrtko
