Re: [PATCH v9 13/13] media: ti: Add CSI2RX support for J721E

[Vinod Koul <vkoul@kernel.org>]

On 04-10-23, 23:03, Laurent Pinchart wrote:
> On Wed, Oct 04, 2023 at 07:21:00PM +0530, Vinod Koul wrote:
> > On 29-08-23, 18:55, Laurent Pinchart wrote:
> > > Hi Jai,
> > > 
> > > (CC'ing Vinod, the maintainer of the DMA engine subsystem, for a
> > > question below)
> > 
> > Sorry this got lost
> 
> No worries.
> 
> > > On Fri, Aug 18, 2023 at 03:55:06PM +0530, Jai Luthra wrote:
> > > > On Aug 15, 2023 at 16:00:51 +0300, Tomi Valkeinen wrote:
> > > > > On 11/08/2023 13:47, Jai Luthra wrote:
> > > > > > From: Pratyush Yadav <p.yadav@ti.com>
> > > 
> > > [snip]
> > > 
> > > > > > +static int ti_csi2rx_start_streaming(struct vb2_queue *vq, unsigned int count)
> > > > > > +{
> > > > > > +	struct ti_csi2rx_dev *csi = vb2_get_drv_priv(vq);
> > > > > > +	struct ti_csi2rx_dma *dma = &csi->dma;
> > > > > > +	struct ti_csi2rx_buffer *buf;
> > > > > > +	unsigned long flags;
> > > > > > +	int ret = 0;
> > > > > > +
> > > > > > +	spin_lock_irqsave(&dma->lock, flags);
> > > > > > +	if (list_empty(&dma->queue))
> > > > > > +		ret = -EIO;
> > > > > > +	spin_unlock_irqrestore(&dma->lock, flags);
> > > > > > +	if (ret)
> > > > > > +		return ret;
> > > > > > +
> > > > > > +	dma->drain.len = csi->v_fmt.fmt.pix.sizeimage;
> > > > > > +	dma->drain.vaddr = dma_alloc_coherent(csi->dev, dma->drain.len,
> > > > > > +					      &dma->drain.paddr, GFP_KERNEL);
> > > > > > +	if (!dma->drain.vaddr)
> > > > > > +		return -ENOMEM;
> > > > > 
> > > > > This is still allocating a large buffer every time streaming is started (and
> > > > > with streams support, a separate buffer for each stream?).
> > > > > 
> > > > > Did you check if the TI DMA can do writes to a constant address? That would
> > > > > be the best option, as then the whole buffer allocation problem goes away.
> > > > 
> > > > I checked with Vignesh, the hardware can support a scenario where we 
> > > > flush out all the data without allocating a buffer, but I couldn't find 
> > > > a way to signal that via the current dmaengine framework APIs. Will look 
> > > > into it further as it will be important for multi-stream support.
> > > 
> > > That would be the best option. It's not immediately apparent to me if
> > > the DMA engine API supports such a use case.
> > > dmaengine_prep_interleaved_dma() gives you finer grain control on the
> > > source and destination increments, but I haven't seen a way to instruct
> > > the DMA engine to direct writes to /dev/null (so to speak). Vinod, is
> > > this something that is supported, or could be supported ?
> > 
> > Write to a dummy buffer could have the same behaviour, no?
> 
> Yes, but if the DMA engine can write to /dev/null, that avoids
> allocating a dummy buffer, which is nicer. For video use cases, dummy
> buffers are often large.

hmmm maybe I haven't comprehended it full, would you mind explaining the
details on how such a potential interleaved transfer would look like so
that we can model it or change apis to model this

-- 
~Vinod