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

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Message-ID: <CAMRc=Md6ucK-TAmtvWMmUGX1KuVE9Wj_z4i7_-Gc7YXP=Omtcw@mail.gmail.com>
Date: Tue, 23 Dec 2025 13:35:30 +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 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.

Bartosz