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Re: RFC: prepare for struct scatterlist entries without page backing

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Hi,

On Wed, Aug 12, 2015 at 10:42 PM, Boaz Harrosh <boaz@plexistor.com> wrote:
> On 08/12/2015 10:05 AM, Christoph Hellwig wrote:
>> 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/
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Thread

RFC: prepare for struct scatterlist entries without page backing Christoph Hellwig <hch@lst.de> - 2015-08-12 09:20 +0200
  [PATCH 22/31] metag: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:20 +0200
  [PATCH 07/31] alpha/pci_iommu: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:20 +0200
  [PATCH 28/31] powerpc: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:20 +0200
  [PATCH 23/31] sh: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:20 +0200
  [PATCH 16/31] s390: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:20 +0200
    Re: [PATCH 16/31] s390: handle page-less SG entries Sebastian Ott <sebott@linux.vnet.ibm.com> - 2015-08-12 14:00 +0200
  [PATCH 11/31] sparc/iommu: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:20 +0200
  [PATCH 03/31] dma-debug: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:30 +0200
  [PATCH 06/31] alpha/pci-noop: handle page-less SG entries Christoph Hellwig <hch@lst.de> - 2015-08-12 09:30 +0200
  [PATCH 02/31] scatterlist: use sg_phys() Christoph Hellwig <hch@lst.de> - 2015-08-12 09:30 +0200
  [PATCH 01/31] scatterlist: add sg_pfn and sg_has_page helpers Christoph Hellwig <hch@lst.de> - 2015-08-12 09:30 +0200
  Re: RFC: prepare for struct scatterlist entries without page backing Boaz Harrosh <boaz@plexistor.com> - 2015-08-12 14:50 +0200
    Re: RFC: prepare for struct scatterlist entries without page backing Julian Calaby <julian.calaby@gmail.com> - 2015-08-13 01:40 +0200
      Re: RFC: prepare for struct scatterlist entries without page  backing Christoph Hellwig <hch@lst.de> - 2015-08-13 16:40 +0200
        Re: RFC: prepare for struct scatterlist entries without page backing Julian Calaby <julian.calaby@gmail.com> - 2015-08-14 01:50 +0200
    Re: RFC: prepare for struct scatterlist entries without page  backing Christoph Hellwig <hch@lst.de> - 2015-08-13 16:50 +0200
      Re: RFC: prepare for struct scatterlist entries without page backing Boaz Harrosh <boaz@plexistor.com> - 2015-08-13 17:50 +0200
  Re: RFC: prepare for struct scatterlist entries without page backing James Bottomley <James.Bottomley@HansenPartnership.com> - 2015-08-12 19:10 +0200
    Re: RFC: prepare for struct scatterlist entries without page backing Grant Grundler <grantgrundler@gmail.com> - 2015-08-12 20:00 +0200

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