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

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Message-ID: <bwhczqghukhoy2ktedizkexwhzdmirrxcucoewrc5dfe7ebvjk@554mr7q2urmr>
Date: Wed, 14 Jan 2026 18:04:51 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Vinod Koul <vkoul@...nel.org>
Cc: Bartosz Golaszewski <brgl@...nel.org>, 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 09, 2026 at 07:57:00AM +0530, Vinod Koul wrote:
> On 02-01-26, 18:14, Bartosz Golaszewski 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.
> 
> I think downstream sends lock descriptor always. What is the harm in
> doing that every time if we go down that path?
> Reg Dmitry question above, this is dma hw capability, how will client
> know if it has to lock on older rev of hardware or not...?

We can identify that on the calling side, but I doubt we'd need it: The
lock semantics was absent on APQ8064 / MSM8960 / IPQ8064 and it seems to
be present for all devices afterwards.

Frankly speaking, I think this is the best API we can get. It is
definitely much better than the original proposal.

> 
> > Also: only the crypto engine needs it for now, not all the other users
> > of the BAM engine.
> 
> But they might eventually right?

-- 
With best wishes
Dmitry