linux-kernel - Re: [PATCH v8 01/11] dmaengine: Add DMA_PREP_LOCK/DMA_PREP

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Message-ID: <CAMRc=MfkVoRGFLSp6gy0aWe_3iA2G5v0U7yvgwLp5JFjmqkzsw@mail.gmail.com>
Date: Thu, 13 Nov 2025 16:52:56 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
Cc: Vinod Koul <vkoul@...nel.org>, Jonathan Corbet <corbet@....net>, 
	Thara Gopinath <thara.gopinath@...il.com>, Herbert Xu <herbert@...dor.apana.org.au>, 
	"David S. Miller" <davem@...emloft.net>, Udit Tiwari <quic_utiwari@...cinc.com>, 
	Daniel Perez-Zoghbi <dperezzo@...cinc.com>, Md Sadre Alam <mdalam@....qualcomm.com>, 
	dmaengine@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	linux-crypto@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v8 01/11] dmaengine: Add DMA_PREP_LOCK/DMA_PREP_UNLOCK flags

On Thu, Nov 13, 2025 at 1:28 PM Dmitry Baryshkov
<dmitry.baryshkov@....qualcomm.com> 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@....qualcomm.com> wrote:
> > >
> > > On Thu, Nov 06, 2025 at 12:33:57PM +0100, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > >
> > > > 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