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

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Message-ID: <66nhvrt4krn7lvmsrqoc5quygh7ckc36fax3fgol2feymqfbdp@lqlfye47cs2p>
Date: Thu, 13 Nov 2025 22:12:35 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Bartosz Golaszewski <brgl@...ev.pl>
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 04:52:56PM +0100, Bartosz Golaszewski wrote:
> 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?

I find it a bit strange that the NS-OS (Linux) can cause side-effects to
the TZ. Please correct me if I'm wrong, but I assumed that TZ should be
able to function even when LInux is misbehaving.

-- 
With best wishes
Dmitry