Hi,

On Wed, Aug 12, 2015 at 10:42 PM, Boaz Harrosh [off-list ref] wrote:

On 08/12/2015 10:05 AM, Christoph Hellwig wrote:

quoted

It turns out most DMA mapping implementation can handle SGLs without
page structures with some fairly simple mechanical work.  Most of it
is just about consistently using sg_phys.  For implementations that
need to flush caches we need a new helper that skips these cache
flushes if a entry doesn't have a kernel virtual address.

However the ccio (parisc) and sba_iommu (parisc & ia64) IOMMUs seem
to be operate mostly on virtual addresses.  It's a fairly odd concept
that I don't fully grasp, so I'll need some help with those if we want
to bring this forward.

Additional this series skips ARM entirely for now.  The reason is
that most arm implementations of the .map_sg operation just iterate
over all entries and call ->map_page for it, which means we'd need
to convert those to a ->map_pfn similar to Dan's previous approach.

[snip]

It is a bit of work but is worth while, and accelerating tremendously
lots of workloads. Not like this abomination which only branches
things more and more, and making things fatter and slower.

As a random guy reading a big bunch of patches on code I know almost
nothing about, parts of this comment really resonated with me:
overall, we seem to be adding a lot of if statements to code that
appears to be in a hot path.

I.e. ~90% of this patch set seems to be just mechanically dropping
BUG_ON()s and converting open coded stuff to use accessor functions
(which should be macros or get inlined, right?) - and the remaining
bit is not flushing if we don't have a physical page somewhere.

Would it make sense to split this patch set into a few bits: one to
drop all the useless BUG_ON()s, one to convert all the open coded
stuff to accessor functions, then another to do the actual page-less
sg stuff?

Thanks,

-- 
Julian Calaby

Email: julian.calaby@gmail.com
Profile: http://www.google.com/profiles/julian.calaby/