Re: [PATCH v5 0/8] iio: new DMABUF based API, v5
From: Andrew Davis
Date: Thu Jan 11 2024 - 12:31:24 EST
On 1/11/24 3:20 AM, Paul Cercueil wrote:
Hi Andrew,
Le lundi 08 janvier 2024 à 15:12 -0600, Andrew Davis a écrit :
On 12/19/23 11:50 AM, Paul Cercueil wrote:
[V4 was: "iio: Add buffer write() support"][1]
Hi Jonathan,
This is a respin of the V3 of my patchset that introduced a new
interface based on DMABUF objects [2].
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.
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):
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.
Have you tried DMABUFs created using the DMABUF System heap exporter?
(drivers/dma-buf/heaps/system_heap.c) It should be able to handle
larger allocation better here, and if you don't have any active
mmaps or vmaps then it can skip CPU-side coherency maintenance
(useful for device to device transfers).
I didn't know about it!
But udmabuf also allows you to skip CPU-side coherency maintenance,
since DMABUFs have two ioctls to start/finish CPU access anyway.
The only way it lets you skip that is if your application just doesn't
call those begin/end ioctls, which is wrong. That may work on a system
where CPU caches can be snooped by all devices that could attach to
a buffer(x86), but that might not work on others(ARM). So calling
those begin/end ioctls is required[0]. If maintenance is not actually
needed then the kernel will turn those calls into NOPs for you, but only
the kernel can know when that is correct (based on the running system
and the devices attached to that buffer), not userspace.
Allocating DMABUFs out of user pages has a bunch of other issues you
might run into also. I'd argue udmabuf is now completely superseded
by DMABUF system heaps. Try it out :)
I'm curious, what other issues?
For starters the {begin,end}_cpu_access() callbacks don't actually
sync the pages for any of the devices attached to the DMABUF, it
only makes a fake mapping for the misc device(CPU) then syncs with
that. That probably works for the QEMU case it was designed for where
the device is always a VM instance running on the same CPU, but for
any real devices the sync never happens towards them.
I have some patches fixing the above I'll post this cycle, but it
wont help with folks doing reads/wrties on the original shmem/memfd
outside of the begin/end ioctls. So there is a fundamental issue
with the buffer's backing memory's ownership/lifecycle that makes
udmabuf broken by design.
The DMABUF System Heap owns the backing memory and manages that
memory's lifecycle as all correct DMABUF exporters must.
The good thing about udmabuf is that the memory is backed by pages, so
we can use MSG_ZEROCOPY on sockets to transfer the mmapped data over
the network (having a DMABUF interface to the network stack would be
better, but I'm not opening that can of worms).
Yes, having a DMABUF importer interface for the network stack would be
the best long-term solution here, and one will probably end up being
needed for zero-copy buffer passing directly between HW and network
which seems to be a growing area of interest. And would help solve
some cases where MSG_ZEROCOPY fails (such as devices without
scatter-gather) by making sure the backing buffer meets the needs
of all attached devices, etc.. But I do agree let's leave those
worm-cans for someone else to open :)
I wonder what would happen if you tried a MSG_ZEROCOPY on a buffer
that was an mmap'd address from a DMABUF.. probably nothing good
but might be worth looking into.
Andrew
[0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/dma-buf/dma-buf.c#n1323
Andrew
Cheers,
-Paul
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.).
Based on linux-next/next-20231219.
Cheers,
-Paul
[1]
https://lore.kernel.org/all/20230807112113.47157-1-paul@xxxxxxxxxxxxxxx/
[2]
https://lore.kernel.org/all/20230403154800.215924-1-paul@xxxxxxxxxxxxxxx/
[3]
https://github.com/analogdevicesinc/libiio/tree/pcercuei/dev-new-dmabuf-api
[4]
https://lore.kernel.org/all/20230322092118.9213-1-paul@xxxxxxxxxxxxxxx/
---
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