Re: [PATCH] media: stm32: dcmi: create a dma scatterlist based on DMA max_sg_burst value

[Alain Volmat <alain.volmat@foss.st.com>]

Hi,

could you put this on hold for the moment, I've noticed that a
sg_free_table is actually missing in this patch, leading to a memory
leak.  I'll thus post a v2 of the patch shortly.

Cheers,
Alain

On Mon, Jan 10, 2022 at 11:37:39AM +0100, Alain Volmat wrote:
> Prior to submitting a transfer to the DMA, the client should first check
> the capabilities of the DMA channel in term of maximum of each segment.
> This is given by the max_sg_burst value reported by dma_get_slave_caps API.
> Based on that, if the transfer is larger than what can be handled by the
> DMA channel, we split the transfer into several scatterlist elements.
> 
> Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
> ---
>  drivers/media/platform/stm32/stm32-dcmi.c | 47 ++++++++++++++++++-----
>  1 file changed, 37 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/media/platform/stm32/stm32-dcmi.c b/drivers/media/platform/stm32/stm32-dcmi.c
> index e1b17c05229c..ee170e999a88 100644
> --- a/drivers/media/platform/stm32/stm32-dcmi.c
> +++ b/drivers/media/platform/stm32/stm32-dcmi.c
> @@ -113,7 +113,7 @@ struct dcmi_framesize {
>  struct dcmi_buf {
>  	struct vb2_v4l2_buffer	vb;
>  	bool			prepared;
> -	dma_addr_t		paddr;
> +	struct sg_table		sgt;
>  	size_t			size;
>  	struct list_head	list;
>  };
> @@ -157,6 +157,7 @@ struct stm32_dcmi {
>  	enum state			state;
>  	struct dma_chan			*dma_chan;
>  	dma_cookie_t			dma_cookie;
> +	u32				dma_max_burst;
>  	u32				misr;
>  	int				errors_count;
>  	int				overrun_count;
> @@ -326,13 +327,11 @@ static int dcmi_start_dma(struct stm32_dcmi *dcmi,
>  	mutex_lock(&dcmi->dma_lock);
>  
>  	/* Prepare a DMA transaction */
> -	desc = dmaengine_prep_slave_single(dcmi->dma_chan, buf->paddr,
> -					   buf->size,
> +	desc = dmaengine_prep_slave_sg(dcmi->dma_chan, buf->sgt.sgl, buf->sgt.nents,
>  					   DMA_DEV_TO_MEM,
>  					   DMA_PREP_INTERRUPT);
>  	if (!desc) {
> -		dev_err(dcmi->dev, "%s: DMA dmaengine_prep_slave_single failed for buffer phy=%pad size=%zu\n",
> -			__func__, &buf->paddr, buf->size);
> +		dev_err(dcmi->dev, "%s: DMA dmaengine_prep_slave_sg failed\n", __func__);
>  		mutex_unlock(&dcmi->dma_lock);
>  		return -EINVAL;
>  	}
> @@ -524,6 +523,10 @@ static int dcmi_buf_prepare(struct vb2_buffer *vb)
>  	struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
>  	struct dcmi_buf *buf = container_of(vbuf, struct dcmi_buf, vb);
>  	unsigned long size;
> +	unsigned int num_sgs = 1;
> +	dma_addr_t dma_buf;
> +	struct scatterlist *sg;
> +	int i, ret;
>  
>  	size = dcmi->fmt.fmt.pix.sizeimage;
>  
> @@ -537,15 +540,33 @@ static int dcmi_buf_prepare(struct vb2_buffer *vb)
>  
>  	if (!buf->prepared) {
>  		/* Get memory addresses */
> -		buf->paddr =
> -			vb2_dma_contig_plane_dma_addr(&buf->vb.vb2_buf, 0);
>  		buf->size = vb2_plane_size(&buf->vb.vb2_buf, 0);
> -		buf->prepared = true;
> +		if (buf->size > dcmi->dma_max_burst)
> +			num_sgs = DIV_ROUND_UP(buf->size, dcmi->dma_max_burst);
>  
> -		vb2_set_plane_payload(&buf->vb.vb2_buf, 0, buf->size);
> +		ret = sg_alloc_table(&buf->sgt, num_sgs, GFP_ATOMIC);
> +		if (ret) {
> +			dev_err(dcmi->dev, "sg table alloc failed\n");
> +			return ret;
> +		}
> +
> +		dma_buf = vb2_dma_contig_plane_dma_addr(&buf->vb.vb2_buf, 0);
>  
>  		dev_dbg(dcmi->dev, "buffer[%d] phy=%pad size=%zu\n",
> -			vb->index, &buf->paddr, buf->size);
> +			vb->index, &dma_buf, buf->size);
> +
> +		for_each_sg(buf->sgt.sgl, sg, num_sgs, i) {
> +			size_t bytes = min_t(size_t, size, dcmi->dma_max_burst);
> +
> +			sg_dma_address(sg) = dma_buf;
> +			sg_dma_len(sg) = bytes;
> +			dma_buf += bytes;
> +			size -= bytes;
> +		}
> +
> +		buf->prepared = true;
> +
> +		vb2_set_plane_payload(&buf->vb.vb2_buf, 0, buf->size);
>  	}
>  
>  	return 0;
> @@ -1866,6 +1887,7 @@ static int dcmi_probe(struct platform_device *pdev)
>  	struct stm32_dcmi *dcmi;
>  	struct vb2_queue *q;
>  	struct dma_chan *chan;
> +	struct dma_slave_caps caps;
>  	struct clk *mclk;
>  	int irq;
>  	int ret = 0;
> @@ -1953,6 +1975,11 @@ static int dcmi_probe(struct platform_device *pdev)
>  		return ret;
>  	}
>  
> +	dcmi->dma_max_burst = UINT_MAX;
> +	ret = dma_get_slave_caps(chan, &caps);
> +	if (!ret && caps.max_sg_burst)
> +		dcmi->dma_max_burst = caps.max_sg_burst	* DMA_SLAVE_BUSWIDTH_4_BYTES;
> +
>  	spin_lock_init(&dcmi->irqlock);
>  	mutex_init(&dcmi->lock);
>  	mutex_init(&dcmi->dma_lock);
> -- 
> 2.25.1
>