Re: [PATCH RFC v11 00/12] crypto/dmaengine: qce: introduce BAM locking and use DMA for register I/O

[Bartosz Golaszewski]

On Thu, Mar 5, 2026 at 5:16 PM Stephan Gerhold
<stephan.gerhold@xxxxxxxxxx> wrote:
>
> >
> > You'd think so but the HPG actually does use the word "dummy" to
> > describe the write operation with lock/unlock bits set. Though it does
> > not recommend any particular register to do it.
> >
>
> I guess the documentation I'm looking at (8.7.3.4 BAM operation in the
> public APQ8016E TRM) might be an excerpt from some older version of the
> BAM HPG. Is also has a note about "dummy" command descriptors:
>
>   "NOTE: Pipe locking and unlocking should appear only in
>    command-descriptor. In case a lock is required on a data descriptor
>    this can be implemented by a dummy command descriptor with
>    lock/unlock bit asserted preceding/following the data descriptor."
>
> This one doesn't make any difference between READ and WRITE command
> descriptors (and both are documented in the chapter).
>
> Personally, I would prefer using a read over a write if possible. Unless
> you can confirm that the register used for the dummy write is actually
> read-only *and* write-ignore, writing to the register is essentially
> undefined behavior. It will probably do the right thing on most
> platforms, but there could also be one out there where writing to the
> register triggers an error or potentially even silently ends up writing
> into another register. Register logic can be fun in practice, commit
> e9a48ea4d90b ("irqchip/qcom-pdc: Workaround hardware register bug on
> X1E80100") [1] is a good example of that. :')
>
> [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e9a48ea4d90be251e0d057d41665745caccb0351

I agree in general but I also learned from the QCE team at Qualcomm
that apparently there were some issues with register reads over DMA,
which makes writes preferable. While the VERSION register is
officially read-only, I've been told writing to it is safe. So it's a
choice between doing a READ that may not work on some platforms and
doing a WRITE that may theoretically not work on some platforms.

I'll send v12 which will be a proper series with using register
metadata and correctly freeing resources and we can rediscuss. Doing
one or the other is actually a minor details of the whole thing after
all.

Bart