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

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Message-ID: <CAMRc=McO-Fbb=O3VjFk5C14CD6oVA4UmLroN4_ddCVxtfxr03A@mail.gmail.com>
Date: Wed, 17 Dec 2025 15:31:41 +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 16, 2025 at 4:11 PM Vinod Koul <vkoul@...nel.org> wrote:
>
> > > > +
> > > > +     switch (metadata->op) {
> > > > +     case BAM_META_CMD_LOCK:
> > > > +             if (bchan->bam_locked)
> > > > +                     return -EBUSY;
> > > > +
> > > > +             hw_desc->flags |= DESC_FLAG_LOCK;
> > >
> > > Why does this flag imply for the hardware.
>
> s/Why/What !
> >
> > Please rephrase, I don't get what you mean.
>
> 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.
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.

As I said: I'm open to other suggestions but I'm not sure if we have
any other existing options.

What exactly is the problem with using the attach callback?

Bart