Re: [PATCH v8 01/11] dmaengine: Add DMA_PREP_LOCK/DMA_PREP_UNLOCK flags

[Bartosz Golaszewski]

On Thu, Nov 13, 2025 at 1:28 PM Dmitry Baryshkov
<dmitry.baryshkov@xxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Nov 13, 2025 at 11:02:11AM +0100, Bartosz Golaszewski wrote:
> > On Tue, Nov 11, 2025 at 1:30 PM Dmitry Baryshkov
> > <dmitry.baryshkov@xxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Thu, Nov 06, 2025 at 12:33:57PM +0100, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> > > >
> > > > Some DMA engines may be accessed from linux and the TrustZone
> > > > simultaneously. In order to allow synchronization, add lock and unlock
> > > > flags for the command descriptor that allow the caller to request the
> > > > controller to be locked for the duration of the transaction in an
> > > > implementation-dependent way.
> > >
> > > What is the expected behaviour if Linux "locks" the engine and then TZ
> > > tries to use it before Linux has a chance to unlock it.
> > >
> >
> > Are you asking about the actual behavior on Qualcomm platforms or are
> > you hinting that we should describe the behavior of the TZ in the docs
> > here? Ideally TZ would use the same synchronization mechanism and not
> > get in linux' way. On Qualcomm the BAM, once "locked" will not fetch
> > the next descriptors on pipes other than the current one until
> > unlocked so effectively DMA will just not complete on other pipes.
> > These flags here however are more general so I'm not sure if we should
> > describe any implementation-specific details.
> >
> > We can say: "The DMA controller will be locked for the duration of the
> > current transaction and other users of the controller/TrustZone will
> > not see their transactions complete before it is unlocked"?
>
> So, basically, we are providing a way to stall TZ's DMA transactions?
> Doesn't sound good enough to me.

Can you elaborate because I'm not sure if you're opposed to the idea
itself or the explanation is not good enough?

Bartosz