[PATCH 0/6] staging: kpc2000_dma: fixes for AIO file-ops.

Matt Sickler Matt.Sickler at daktronics.com
Tue Jun 11 23:43:19 UTC 2019

>-----Original Message-----
>From: Jeremy Sowden <jeremy at azazel.net>
>I've had a go at getting the DMA AIO working.  It compiles, but I don't
>have access to the hardware, so I have done no more testing than that.

Honestly, it'd probably be better to just remove the AIO support entirely.  The
one use case we had that could possibly have benefitted from AIO has been
switched away from DMA entirely.  We switched because the DMA buffer was a
couple hundred bytes and the overhead of setting up the DMA was killing
throughput.  AIO *might* have been able to help there, but the userspace side
of AIO is a PITA to work with.  IMO, if "AIO" for the kpc_dma driver were to
make a come back, I think it would be better to use something like io_uring
The other things that use DMA wouldn't benefit from AIO as they have to setup
other parts of the hardware that can't coordinate with the DMA controllers
(or at least not without a lot of work).

TL;DR: it's probably better to just kill the AIO parts of the driver than to try to make them work.

>The fifth patch removes the cancel call-back because it is empty and so
>doesn't serve any purpose (AFAICS).  However, it doesn't appear to be
>too tricky to implement something that would abort the transfer in the
>same manner that kpc_dma_close() if this would be useful.

It's empty because I didn't have time to figure out how to cancel the DMA
operation on the hardware side.  Doing the same "reset the whole engine"
type of cancel could work, but I'm not sure how well that would mesh with
aio_cancel (the latter would kill *all* in-flight operations, the former
is only killing the one).  As I said above, it's probably better to just
remove all the AIO pieces.

>Jeremy Sowden (6):
>  staging: kpc2000_dma: added Kconfig to enable asynchronous I/O.
>  staging: kpc2000_dma: removed casts of void pointers.
>  staging: kpc2000_dma: formatting fixes for AIO functions.
>  staging: kpc2000_dma: replaced aio_(read|write) file-ops with
>    (read|write)_iter ones.
>  staging: kpc2000_dma: removed aio cancel call-back.
>  staging: kpc2000: updated TODO in light of DMA AIO fixes.
> drivers/staging/kpc2000/Kconfig           |  8 +++
> drivers/staging/kpc2000/TODO              |  4 +-
> drivers/staging/kpc2000/kpc_dma/fileops.c | 69 ++++++++++++-----------
> 3 files changed, 44 insertions(+), 37 deletions(-)

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