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

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Message-ID: <whxi2ikode53vrxqpanryw74zd7oovfielgdvhpkka5zy76g75@dxreidnb77y5>
Date: Fri, 21 Nov 2025 18:36:34 +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 Fri, Nov 21, 2025 at 03:35:50PM +0100, Bartosz Golaszewski wrote:
> On Thu, Nov 13, 2025 at 9:12 PM Dmitry Baryshkov
> <dmitry.baryshkov@....qualcomm.com> wrote:
> >
> > 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.
> >
> 
> Ok, so the consensus after talking to Qualcomm crypto engineers - and
> I understand this is Qualcomm-specific but it should apply to any
> similar use-cases - is this:
> 
> If the TZ uses BAM locking and it locks the BAM and linux tries to
> write to the registers protected by this lock, we'll get an external
> abort. Making linux use it too addresses that potential problem.
> 
> Linux could potentially lock and never unlock the BAM but TZ could
> also just reset it. Also: linux could as well turn the entire device
> off. :)
> 
> For the Qualcomm use-case this is not an issue - it's about making TZ
> and linux work together. I suppose the same would apply to any other
> users.

Ack, thank you.

> 
> If that could be contained within the crypto driver, there would be no
> issue. It's just that in order to pass this bit to the DMA controller,
> we need a generic flag. If you have better suggestions, please let me
> know.
> 
> The flag has to be passed to the BAM driver at the time of calling of
> dmaengine_prep_slave_sg() and attrs seems to be the only way with the
> current interface. Off the top of my head: we could extend struct
> scatterlist to allow passing some arbitrary driver data but that
> doesn't sound like a good approach.

Can we use DMA metadata in order to pass the lock / unlock flags
instead? I might be missing something, but the LOCK / UNLOCK ops defined
in this patchset seem to be too usecase-specific. Using metadata seems
to allow for this kind of driver-specific sidechannel.

-- 
With best wishes
Dmitry