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

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Message-ID: <aVf5WUe9cAXZHxPJ@vaman>
Date: Fri, 2 Jan 2026 22:29:05 +0530
From: Vinod Koul <vkoul@...nel.org>
To: Bartosz Golaszewski <brgl@...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 02-01-26, 10:26, Bartosz Golaszewski wrote:
> 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.

Okay then why do we need an API?

Just lock it always and set the bits in the dma driver

-- 
~Vinod