linux-kernel - Re: [PATCH v9 03/11] dmaengine: qcom: bam_dma: implement support for BAM locking

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Message-ID: <CAMRc=Md8nAVWvKK=Vib7TKVzC15M4FmET7TCCdrdS74DKQQjzg@mail.gmail.com>
Date: Wed, 24 Dec 2025 09:58:05 +0100
From: Bartosz Golaszewski <brgl@...nel.org>
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>, 
	Dmitry Baryshkov <lumag@...nel.org>, 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 v9 03/11] dmaengine: qcom: bam_dma: implement support for
 BAM locking

On Tue, Dec 23, 2025 at 9:19 PM Dmitry Baryshkov
<dmitry.baryshkov@....qualcomm.com> wrote:
>
> On Tue, Dec 23, 2025 at 01:35:30PM +0100, Bartosz Golaszewski wrote:
> > On Tue, Dec 23, 2025 at 11:45 AM Vinod Koul <vkoul@...nel.org> wrote:
> > >
> > > On 17-12-25, 15:31, Bartosz Golaszewski wrote:
> > > > On Tue, Dec 16, 2025 at 4:11 PM Vinod Koul <vkoul@...nel.org> wrote:
> > >
> > > > >
> > > > > I am trying to understand what the flag refers to and why do you need
> > > > > this.. What is the problem that lock tries to solve
> > > > >
> > > >
> > > > In the DRM use-case the TA will use the QCE simultaneously with linux.
> > >
> > > TA..?
> >
> > Trusted Application, the one to which we offload the decryption of the
> > stream. That's not really relevant though.
> >
> > >
> > > > It will perform register I/O with DMA using the BAM locking mechanism
> > > > for synchronization. Currently linux doesn't use BAM locking and is
> > > > using CPU for register I/O so trying to access locked registers will
> > > > result in external abort. I'm trying to make the QCE driver use DMA
> > > > for register I/O AND use BAM locking. To that end: we need to pass
> > > > information about wanting the command descriptor to contain the
> > > > LOCK/UNLOCK flag (this is what we set here in the hardware descriptor)
> > > > from the QCE driver to the BAM driver. I initially used a global flag.
> > > > Dmitry said it's too Qualcomm-specific and to use metadata instead.
> > > > This is what I did in this version.
> > >
> > > Okay, how will client figure out should it set the lock or not? What are
> > > the conditions where the lock is set or not set by client..?
> > >
> >
> > I'm not sure what you refer to as "client". The user of the BAM engine
> > - the crypto driver? If so - we convert it to always lock/unlock
> > assuming the TA *may* use it and it's better to be safe. Other users
> > are not affected.
>
> Just to confirm, we support QCE since IPQ4019 and MSM8996. Is lock
> semantics supported on those platforms?
>

Yes, locking is supported on BAM since version 1.4. The only user of
this feature right now is the crypto engine and even on IPQ4019 and
MSM8996 the crypto BAM is version 1.7.

Bartosz