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

[Nuno Sá]

On Mon, 2023-08-07 at 13:21 +0200, Paul Cercueil 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.
> 
> 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, 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.
> 
> 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.
> 

Yeah, I've started that process last week:

https://lore.kernel.org/linux-iio/20230804145342.1600136-1-nuno.sa@xxxxxxxxxx/

the dac counterpart is actually missing in the RFC (as the goal of the RFC is
not review those drivers but the framework) but I do state that my plan is to
have it in the actual series where I actually mention we would need this work
:).

- Nuno Sá