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

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Message-ID: <CAMRc=McyTAvshqgfwTYpN1Av3Z4K=udzrr5t12fwcsBc=vtrcA@mail.gmail.com>
Date: Tue, 6 Jan 2026 13:20:12 +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 Fri, Jan 2, 2026 at 6:14 PM Bartosz Golaszewski <brgl@...nel.org> wrote:
>
> On Fri, Jan 2, 2026 at 5:59 PM Vinod Koul <vkoul@...nel.org> wrote:
> >
> > 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
> >
>
> We need an API because we send a locking descriptor, then a regular
> descriptor (or descriptors) for the actual transaction(s) and then an
> unlocking descriptor. It's a thing the user of the DMA engine needs to
> decide on, not the DMA engine itself.
>
> Also: only the crypto engine needs it for now, not all the other users
> of the BAM engine.
>

Hi Vinod, is there anything else I can do or more information I can
provide in order to move this forward?

Bartosz