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

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Message-ID: <CAMRc=MetbSuaU9VpK7CTio4kt-1pkwEFecARv7ROWDH_yq63OQ@mail.gmail.com>
Date: Tue, 16 Dec 2025 16:00:51 +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 2:00 PM Vinod Koul <vkoul@...nel.org> wrote:
>
> On 28-11-25, 12:44, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> >
> > Use metadata operations in DMA descriptors to allow BAM users to pass
> > additional information to the engine. To that end: define a new
> > structure - struct bam_desc_metadata - as a medium and define two new
> > commands: for locking and unlocking the BAM respectively. Handle the
> > locking in the .attach() callback.
> >
> > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > ---
> >  drivers/dma/qcom/bam_dma.c       | 59 +++++++++++++++++++++++++++++++++++++++-
> >  include/linux/dma/qcom_bam_dma.h | 12 ++++++++
> >  2 files changed, 70 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/dma/qcom/bam_dma.c b/drivers/dma/qcom/bam_dma.c
> > index c9ae1fffe44d79c5eb59b8bbf7f147a8fa3aa0bd..d1dc80b29818897b333cd223ec7306a169cc51fd 100644
> > --- a/drivers/dma/qcom/bam_dma.c
> > +++ b/drivers/dma/qcom/bam_dma.c
> > @@ -30,6 +30,7 @@
> >  #include <linux/module.h>
> >  #include <linux/interrupt.h>
> >  #include <linux/dma-mapping.h>
> > +#include <linux/dma/qcom_bam_dma.h>
> >  #include <linux/scatterlist.h>
> >  #include <linux/device.h>
> >  #include <linux/platform_device.h>
> > @@ -391,6 +392,8 @@ struct bam_chan {
> >       struct list_head desc_list;
> >
> >       struct list_head node;
> > +
> > +     bool bam_locked;
> >  };
> >
> >  static inline struct bam_chan *to_bam_chan(struct dma_chan *common)
> > @@ -655,6 +658,53 @@ static int bam_slave_config(struct dma_chan *chan,
> >       return 0;
> >  }
> >
> > +static int bam_metadata_attach(struct dma_async_tx_descriptor *desc, void *data, size_t len)
> > +{
> > +     struct virt_dma_desc *vd = container_of(desc, struct virt_dma_desc, tx);
> > +     struct bam_async_desc *async_desc = container_of(vd, struct bam_async_desc,  vd);
> > +     struct bam_desc_hw *hw_desc = async_desc->desc;
> > +     struct bam_desc_metadata *metadata = data;
> > +     struct bam_chan *bchan = to_bam_chan(metadata->chan);
> > +     struct bam_device *bdev = bchan->bdev;
> > +
> > +     if (!data)
> > +             return -EINVAL;
> > +
> > +     if (metadata->op == BAM_META_CMD_LOCK || metadata->op == BAM_META_CMD_UNLOCK) {
> > +             if (!bdev->dev_data->bam_pipe_lock)
> > +                     return -EOPNOTSUPP;
> > +
> > +             /* Expecting a dummy write when locking, only one descriptor allowed. */
> > +             if (async_desc->num_desc != 1)
> > +                     return -EINVAL;
> > +     }
> > +
> > +     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.

Please rephrase, I don't get what you mean.

>
> I do not like the interface designed here. This is overloading. Can we
> look at doing something better here.
>

It used to be a generic flag in dmaengine visible for all users.
Dmitry argued that it's too Qualcomm-specific for a generic flag and
suggested using the metadata to hide the communication between the QCE
and BAM drivers. I'm open to other suggestions but it has to be a bit
more specific than "do something better". :)

Bartosz