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

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Message-ID: <CAMRc=MePAVMZPju6rZsyQMir4CkQi+FEqbC++omQtVQC1rHBVg@mail.gmail.com>
Date: Fri, 2 Jan 2026 10:26:49 +0100
From: Bartosz Golaszewski <brgl@...nel.org>
To: Vinod Koul <vkoul@...nel.org>
Cc: 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 Thu, Jan 1, 2026 at 1:00 PM Vinod Koul <vkoul@...nel.org> wrote:
>
> > >
> > > > 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.
>
> Client are users of dmaengine. So how does the crypto driver figure out
> when to lock/unlock. Why not do this always...?
>

It *does* do it always. We assume the TA may be doing it so the crypto
driver is converted to *always* perform register I/O with DMA *and* to
always lock the BAM for each transaction later in the series. This is
why Dmitry inquired whether all the HW with upstream support actually
supports the lock semantics.

Bart