Subject: Re: [PATCH RFC v11 12/12] dmaengine: qcom: bam_dma: add support for BAM locking

From: Vinod Koul <[email-masked]>
Date: Sun, 8 Mar 2026 02:03:22 +0530

On 04-03-26, 16:27, Bartosz Golaszewski wrote:
> On Wed, Mar 4, 2026 at 3:53 PM Vinod Koul <[email-masked]> wrote:
> >
> > On 02-03-26, 16:57, Bartosz Golaszewski wrote:
> > > Add support for BAM pipe locking. To that end: when starting the DMA on
> > > an RX channel - wrap the already issued descriptors with additional
> > > command descriptors performing dummy writes to the base register
> > > supplied by the client via dmaengine_slave_config() (if any) alongside
> > > the lock/unlock HW flags.
> > >
> > > Signed-off-by: Bartosz Golaszewski <[email-masked]>
> 
> [snip]
> 
> > > +static struct bam_async_desc *
> > > +bam_make_lock_desc(struct bam_chan *bchan, struct scatterlist *sg,
> > > +                struct bam_cmd_element *ce, unsigned int flag)
> > > +{
> > > +     struct dma_chan *chan = &bchan->vc.chan;
> > > +     struct bam_async_desc *async_desc;
> > > +     struct bam_desc_hw *desc;
> > > +     struct virt_dma_desc *vd;
> > > +     struct virt_dma_chan *vc;
> > > +     unsigned int mapped;
> > > +     dma_cookie_t cookie;
> > > +     int ret;
> > > +
> > > +     async_desc = kzalloc_flex(*async_desc, desc, 1, GFP_NOWAIT);
> > > +     if (!async_desc) {
> > > +             dev_err(bchan->bdev->dev, "failed to allocate the BAM lock descriptor\n");
> > > +             return NULL;
> > > +     }
> > > +
> > > +     async_desc->num_desc = 1;
> > > +     async_desc->curr_desc = async_desc->desc;
> > > +     async_desc->dir = DMA_MEM_TO_DEV;
> > > +
> > > +     desc = async_desc->desc;
> > > +
> > > +     bam_prep_ce_le32(ce, bchan->slave.dst_addr, BAM_WRITE_COMMAND, 0);
> > > +     sg_set_buf(sg, ce, sizeof(*ce));
> > > +
> > > +     mapped = dma_map_sg_attrs(chan->slave, sg, 1, DMA_TO_DEVICE, DMA_PREP_CMD);
> > > +     if (!mapped) {
> > > +             kfree(async_desc);
> > > +             return NULL;
> > > +     }
> > > +
> > > +     desc->flags |= cpu_to_le16(DESC_FLAG_CMD | flag);
> > > +     desc->addr = sg_dma_address(sg);
> > > +     desc->size = sizeof(struct bam_cmd_element);
> > > +
> > > +     vc = &bchan->vc;
> > > +     vd = &async_desc->vd;
> > > +
> > > +     dma_async_tx_descriptor_init(&vd->tx, &vc->chan);
> > > +     vd->tx.flags = DMA_PREP_CMD;
> > > +     vd->tx.desc_free = vchan_tx_desc_free;
> > > +     vd->tx_result.result = DMA_TRANS_NOERROR;
> > > +     vd->tx_result.residue = 0;
> > > +
> > > +     cookie = dma_cookie_assign(&vd->tx);
> > > +     ret = dma_submit_error(cookie);
> >
> > I am not sure I understand this.
> >
> > At start you add a descriptor in the queue, ideally which should be
> > queued after the existing descriptors are completed!
> >
> > Also I thought you want to append Pipe cmd to descriptors, why not do
> > this while preparing the descriptors and add the pipe cmd and start and
> > end of the sequence when you prepare... This was you dont need to create
> > a cookie like this
> >
> 
> Client (in this case - crypto engine) can call
> dmaengine_prep_slave_sg() multiple times adding several logical
> descriptors which themselves can have several hardware descriptors. We
> want to lock the channel before issuing the first queued descriptor
> (for crypto: typically data descriptor) and unlock it once the final
> descriptor is processed (typically command descriptor). To that end:
> we insert the dummy command descriptor with the lock flag at the head
> of the queue and the one with the unlock flag at the tail - "wrapping"
> the existing queue with lock/unlock operations.

Why not do this per prep call submitted to the engine. It would be
simpler to just add lock and unlock to the start and end of transaction.

-- 
~Vinod