Re: [PATCH v5 0/8] iio: new DMABUF based API, v5

[Paul Cercueil <paul@crapouillou.net>]

Hi Jonathan,

Le jeudi 21 décembre 2023 à 16:30 +0000, Jonathan Cameron a écrit :
> On Tue, 19 Dec 2023 18:50:01 +0100
> Paul Cercueil <paul@crapouillou.net> wrote:
> 
> > [V4 was: "iio: Add buffer write() support"][1]
> > 
> > Hi Jonathan,
> > 
> Hi Paul,
> 
> > This is a respin of the V3 of my patchset that introduced a new
> > interface based on DMABUF objects [2].
> 
> Great to see this moving forwards.
> 
> > 
> > The V4 was a split of the patchset, to attempt to upstream buffer
> > write() support first. But since there is no current user upstream,
> > it
> > was not merged. This V5 is about doing the opposite, and contains
> > the
> > new DMABUF interface, without adding the buffer write() support. It
> > can
> > already be used with the upstream adi-axi-adc driver.
> 
> Seems like a sensible path in the short term.
> 
> > 
> > In user-space, Libiio uses it to transfer back and forth blocks of
> > samples between the hardware and the applications, without having
> > to
> > copy the data.
> > 
> > On a ZCU102 with a FMComms3 daughter board, running Libiio from the
> > pcercuei/dev-new-dmabuf-api branch [3], compiled with
> > WITH_LOCAL_DMABUF_API=OFF (so that it uses fileio):
> >   sudo utils/iio_rwdev -b 4096 -B cf-ad9361-lpc
> >   Throughput: 116 MiB/s
> > 
> > Same hardware, with the DMABUF API (WITH_LOCAL_DMABUF_API=ON):
> >   sudo utils/iio_rwdev -b 4096 -B cf-ad9361-lpc
> >   Throughput: 475 MiB/s
> > 
> > This benchmark only measures the speed at which the data can be
> > fetched
> > to iio_rwdev's internal buffers, and does not actually try to read
> > the
> > data (e.g. to pipe it to stdout). It shows that fetching the data
> > is
> > more than 4x faster using the new interface.
> > 
> > When actually reading the data, the performance difference isn't
> > that
> > impressive (maybe because in case of DMABUF the data is not in
> > cache):
> 
> This needs a bit more investigation ideally. Perhaps perf counters
> can be
> used to establish that cache misses are the main different between
> dropping it on the floor and actually reading the data.

Yes, we'll work on it. The other big difference is that fileio uses
dma_alloc_coherent() while the DMABUFs use non-coherent mappings. I
guess coherent memory is faster for the typical access pattern (which
is "read/write everything sequentially once").

> > 
> > WITH_LOCAL_DMABUF_API=OFF (so that it uses fileio):
> >   sudo utils/iio_rwdev -b 4096 cf-ad9361-lpc | dd of=/dev/zero
> > status=progress
> >   2446422528 bytes (2.4 GB, 2.3 GiB) copied, 22 s, 111 MB/s
> > 
> > WITH_LOCAL_DMABUF_API=ON:
> >   sudo utils/iio_rwdev -b 4096 cf-ad9361-lpc | dd of=/dev/zero
> > status=progress
> >   2334388736 bytes (2.3 GB, 2.2 GiB) copied, 21 s, 114 MB/s
> > 
> > One interesting thing to note is that fileio is (currently)
> > actually
> > faster than the DMABUF interface if you increase a lot the buffer
> > size.
> > My explanation is that the cache invalidation routine takes more
> > and
> > more time the bigger the DMABUF gets. This is because the DMABUF is
> > backed by small-size pages, so a (e.g.) 64 MiB DMABUF is backed by
> > up
> > to 16 thousands pages, that have to be invalidated one by one. This
> > can
> > be addressed by using huge pages, but the udmabuf driver does not
> > (yet)
> > support creating DMABUFs backed by huge pages.
> 
> I'd imagine folios of reasonable size will help sort of a huge page
> as then hopefully it will use the flush by va range instructions if
> available.
> 
> > 
> > Anyway, the real benefits happen when the DMABUFs are either shared
> > between IIO devices, or between the IIO subsystem and another
> > filesystem. In that case, the DMABUFs are simply passed around
> > drivers,
> > without the data being copied at any moment.
> > 
> > We use that feature to transfer samples from our transceivers to
> > USB,
> > using a DMABUF interface to FunctionFS [4].
> > 
> > This drastically increases the throughput, to about 274 MiB/s over
> > a
> > USB3 link, vs. 127 MiB/s using IIO's fileio interface + write() to
> > the
> > FunctionFS endpoints, for a lower CPU usage (0.85 vs. 0.65 load
> > avg.).
> 
> This is a nice example.  Where are you with getting the patch merged?

I'll send a new version (mostly a [RESEND]...) in the coming days. As
you can see from the review on my last attempt, the main blocker is
that nobody wants to merge a new interface if the rest of the kernel
bits aren't upstream yet. Kind of a chicken-and-egg problem :)

> Overall, this code looks fine to me, though there are some parts that
> need review by other maintainers (e.g. Vinod for the dmaengine
> callback)
> and I'd like a 'looks fine' at least form those who know a lot more
> about dmabuf than I do.
> 
> To actually make this useful sounds like either udmabuf needs some
> perf
> improvements, or there has to be an upstream case of sharing it
> without
> something else (e.g your functionfs patches).  So what do we need to
> get in before the positive benefit becomes worth carrying this extra
> complexity? (which isn't too bad so I'm fine with a small benefit and
> promises of riches :)

I think the FunctionFS DMABUF interface can be pushed as well for 5.9,
in parallel of this one, as the feedback on the V1 was good. I might
just need some help pushing it forward (kind of a "I merge it if you
merge it" guarantee).

Cheers,
-Paul

> 
> Jonathan
> 
> > 
> > Based on linux-next/next-20231219.
> > 
> > Cheers,
> > -Paul
> > 
> > [1]
> > https://lore.kernel.org/all/20230807112113.47157-1-paul@crapouillou.net/
> > [2]
> > https://lore.kernel.org/all/20230403154800.215924-1-paul@crapouillou.net/
> > [3]
> > https://github.com/analogdevicesinc/libiio/tree/pcercuei/dev-new-dmabuf-api
> > [4]
> > https://lore.kernel.org/all/20230322092118.9213-1-paul@crapouillou.net/
> > 
> > ---
> > Changelog:
> > - [3/8]: Replace V3's dmaengine_prep_slave_dma_array() with a new
> >   dmaengine_prep_slave_dma_vec(), which uses a new 'dma_vec'
> > struct.
> >   Note that at some point we will need to support cyclic transfers
> >   using dmaengine_prep_slave_dma_vec(). Maybe with a new "flags"
> >   parameter to the function?
> > 
> > - [4/8]: Implement .device_prep_slave_dma_vec() instead of V3's
> >   .device_prep_slave_dma_array().
> > 
> >   @Vinod: this patch will cause a small conflict with my other
> >   patchset adding scatter-gather support to the axi-dmac driver.
> >   This patch adds a call to axi_dmac_alloc_desc(num_sgs), but the
> >   prototype of this function changed in my other patchset - it
> > would
> >   have to be passed the "chan" variable. I don't know how you
> > prefer it
> >   to be resolved. Worst case scenario (and if @Jonathan is okay
> > with
> >   that) this one patch can be re-sent later, but it would make this
> >   patchset less "atomic".
> > 
> > - [5/8]:
> >   - Use dev_err() instead of pr_err()
> >   - Inline to_iio_dma_fence()
> >   - Add comment to explain why we unref twice when detaching dmabuf
> >   - Remove TODO comment. It is actually safe to free the file's
> >     private data even when transfers are still pending because it
> >     won't be accessed.
> >   - Fix documentation of new fields in struct
> > iio_buffer_access_funcs
> >   - iio_dma_resv_lock() does not need to be exported, make it
> > static
> > 
> > - [7/8]:
> >   - Use the new dmaengine_prep_slave_dma_vec().
> >   - Restrict to input buffers, since output buffers are not yet
> >     supported by IIO buffers.
> > 
> > - [8/8]:
> >   Use description lists for the documentation of the three new
> > IOCTLs
> >   instead of abusing subsections.
> > 
> > ---
> > Alexandru Ardelean (1):
> >   iio: buffer-dma: split iio_dma_buffer_fileio_free() function
> > 
> > Paul Cercueil (7):
> >   iio: buffer-dma: Get rid of outgoing queue
> >   dmaengine: Add API function dmaengine_prep_slave_dma_vec()
> >   dmaengine: dma-axi-dmac: Implement device_prep_slave_dma_vec
> >   iio: core: Add new DMABUF interface infrastructure
> >   iio: buffer-dma: Enable support for DMABUFs
> >   iio: buffer-dmaengine: Support new DMABUF based userspace API
> >   Documentation: iio: Document high-speed DMABUF based API
> > 
> >  Documentation/iio/dmabuf_api.rst              |  54 +++
> >  Documentation/iio/index.rst                   |   2 +
> >  drivers/dma/dma-axi-dmac.c                    |  40 ++
> >  drivers/iio/buffer/industrialio-buffer-dma.c  | 242 ++++++++---
> >  .../buffer/industrialio-buffer-dmaengine.c    |  52 ++-
> >  drivers/iio/industrialio-buffer.c             | 402
> > ++++++++++++++++++
> >  include/linux/dmaengine.h                     |  25 ++
> >  include/linux/iio/buffer-dma.h                |  33 +-
> >  include/linux/iio/buffer_impl.h               |  26 ++
> >  include/uapi/linux/iio/buffer.h               |  22 +
> >  10 files changed, 836 insertions(+), 62 deletions(-)
> >  create mode 100644 Documentation/iio/dmabuf_api.rst
> > 
>