Re: [PATCH v4 0/6] iio: Add buffer write() support

[Nuno Sá]

On Wed, 2023-08-30 at 17:18 +0100, Jonathan Cameron wrote:
> On Wed, 30 Aug 2023 17:11:18 +0100
> Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
> 
> > On Mon,  7 Aug 2023 13:21:07 +0200
> > Paul Cercueil <paul@xxxxxxxxxxxxxxx> wrote:
> > 
> > > [V3 was: "iio: new DMABUF based API, v3"][1]
> > > 
> > > Hi Jonathan,
> > > 
> > > This is a subset of my patchset that introduced a new interface based on
> > > DMABUF objects [1]. It adds write() support to the IIO buffer
> > > infrastructure.
> > > 
> > > The reason it is not the full IIO-DMABUF patchset, is because you
> > > requested performance benchmarks - and our current numbers are barely
> > > better (~ +10%) than the fileio interface. There is a good reason for
> > > that: V3 of the patchset switched from having the IIO core creating the
> > > DMABUFs backed by physically contiguous memory, to having the IIO core
> > > being a simple DMABUF importer, and having the DMABUFs created
> > > externally. We now use the udmabuf driver to create those, and they are
> > > allocated from paged memory. While this works perfectly fine, our
> > > buffers are now cut in 4 KiB chunks (pages), non-contiguous in memory,
> > > which causes the DMA hardware to create an IRQ storm, as it raises an
> > > interrupt after each 4 KiB in the worst case scenario.  
> > 
> > Interesting. I'm guessing you don't necessarily need contiguous memory
> > and huge pages would get rid of most of that overhead?
> > 
> > Given embedded target those huge pages are hard to get so you need
> > hugetlb support to improve the chances of it working.  Some quick searching
> > suggests there is possible support on the way.
> > https://lore.kernel.org/linux-mm/20230817064623.3424348-1-vivek.kasireddy@xxxxxxxxx/
> > 
> > 
> > > 
> > > Anyway, this is not directly a problem of the IIO-DMABUF code - but I
> > > can't really upstream a shiny new interface that I claim is much faster,
> > > without giving numbers.
> > > 
> > > So while we fix this (either by updating the DMA IP and driver to
> > > support scatter-gather)  
> > 
> > Long run you almost always end up needing that unless contig requirements
> > are small and you want a robust solution.  I'm guessing no IOMMU to pretend
> > it's all contiguous... 
> > 
> > > or by hacking something quick to give us
> > > physically contiguous DMABUFs just for the benchmark), I thought it
> > > would make sense to upstream the few patches of the V3 patchset that are
> > > needed for the IIO-DMABUF interface but aren't directly related.  
> > 
> > Good idea.
> > 
> > > 
> > > As for write() support, Nuno (Cc'd) said he will work on upstreaming the
> > > DAC counterpart of adc/adi-axi-adc.c in the next few weeks, so there
> > > will be a user for the buffer write() support. I hope you are okay with
> > > this - otherwise, we can just wait until this work is done and submit it
> > > all at once.  
> > 
> > Absolutely fine, though I won't pick this up without the user also being
> > ready to go.
> 
> 
> Having looked through these again, they are straight forward so no changes
> requested from me.  Nuno, if you can add this set into appropriate
> point in your series that will make use of it that will make my life easier
> and ensure and minor rebasing etc happens without having to bother Paul.
> 

Sure...

- Nuno Sá
>