Date: Sun, 26 Jul 2015 13:36:58 +0200 (CEST)
From: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
To: Robert Jarzmik <robert.jarzmik@free.fr>
cc: Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
        Jiri Kosina <trivial@kernel.org>, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/4] media: pxa_camera: conversion to dmaengine
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Hi Robert,

On Sun, 19 Jul 2015, Robert Jarzmik wrote:

> Robert Jarzmik <robert.jarzmik@free.fr> writes:
> 
> > Guennadi Liakhovetski <g.liakhovetski@gmx.de> writes:
> >
> >>>  		/* init DMA for Y channel */
> >>
> >> How about taking the loop over the sg list out of pxa_init_dma_channel() 
> >> to avoid having to iterate it from the beginning each time? Then you would 
> >> be able to split it into channels inside that global loop? Would that 
> >> work? Of course you might need to rearrange functions to avoid too deep 
> >> code nesting.
> >
> > Ok, will try that.
> > The more I think of it, the more it looks to me like a generic thing : take an
> > sglist, and an array of sizes, and split the sglist into several sglists, each
> > of the defined size in the array.
> >
> > Or more code-like speaking :
> >   - sglist_split(struct scatterlist *sg_int, size_t *sizes, int nb_sizes,
> >                  struct scatterlist **sg_out)
> >   - and sg_out is an array of nb_sizes (struct scatterlist *sg)
> >
> > So I will try that out. Maybe if that works out for pxa_camera, Jens or Russell
> > would accept that into lib/scatterlist.c.
> Ok, I made the code ... and I hate it.
> It's in [1], which is an incremental patch over patch 4/4.

It would've been easier to review, if it were on top of the mainline, 
resp. your patches 1-3 from this series.

> If that's what you had in mind, tell me.

In principle, yes, it doesn't look all that horrible to me. You first 
split the global SG list into up to 3 per-channel ones, then you 
initialise your channels, what's wrong with that? Just have to add some 
polish to it here and there... This is a preliminary review, I'll do a 
proper one, once you fix these and send me anew version, not based on top 
of patch 4/4.

> Cheers.
> 
> --
> Robert
> 
> [1] The despised patch
> ---<8---
> commit 43bbb9a4e3ac
> Author: Robert Jarzmik <robert.jarzmik@free.fr>
> Date:   Tue Jul 14 20:17:51 2015 +0200
> 
>     tmp: pxa_camera: working on sg_split
>     
>     Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
> 
> diff --git a/drivers/media/platform/soc_camera/pxa_camera.c b/drivers/media/platform/soc_camera/pxa_camera.c
> index 26a66b9ff570..83efd284e976 100644
> --- a/drivers/media/platform/soc_camera/pxa_camera.c
> +++ b/drivers/media/platform/soc_camera/pxa_camera.c
> @@ -287,64 +287,110 @@ static void free_buffer(struct videobuf_queue *vq, struct pxa_buffer *buf)
>  		&buf->vb, buf->vb.baddr, buf->vb.bsize);
>  }
>  
> -static struct scatterlist *videobuf_sg_cut(struct scatterlist *sglist,
> -					   int sglen, int offset, int size,
> -					   int *new_sg_len)
> +
> +struct sg_splitter {
> +	struct scatterlist *in_sg0;
> +	int nents;
> +	off_t skip_sg0;
> +	size_t len_last_sg;
> +	struct scatterlist *out_sg;
> +};
> +
> +static struct sg_splitter *
> +sg_calculate_split(struct scatterlist *in, off_t skip,

You don't need "skip," you only call this function once with skip == 0. 
Besides I usually prefer all the keywords before the function name, the 
function name, the opening parenthesis and at least the first argument on 
the same line. So far pxa_camera.c follows this, let's keep it this way. 
It makes grepping for functions easier. And no, I don't care about 80 
chars...

> +		   const size_t *sizes, int nb_splits, gfp_t gfp_mask)
>  {
> -	struct scatterlist *sg0, *sg, *sg_first = NULL;
> -	int i, dma_len, dropped_xfer_len, dropped_remain, remain;
> -	int nfirst = -1, nfirst_offset = 0, xfer_len;
> -
> -	*new_sg_len = 0;
> -	dropped_remain = offset;
> -	remain = size;
> -	for_each_sg(sglist, sg, sglen, i) {
> -		dma_len = sg_dma_len(sg);
> -		/* PXA27x Developer's Manual 27.4.4.1: round up to 8 bytes */
> -		dropped_xfer_len = roundup(min(dma_len, dropped_remain), 8);
> -		if (dropped_remain)
> -			dropped_remain -= dropped_xfer_len;
> -		xfer_len = dma_len - dropped_xfer_len;
> -
> -		if (nfirst < 0 && xfer_len > 0) {
> -			sg_first = sg;
> -			nfirst = i;
> -			nfirst_offset = dropped_xfer_len;
> +	int i, nents;
> +	size_t size, len;
> +	struct sg_splitter *splitters, *curr;
> +	struct scatterlist *sg;
> +
> +	splitters = kcalloc(nb_splits, sizeof(*splitters), gfp_mask);

This is an array of at most 3 elements, 20 bytes each. I'd just allocate 
it on stack in the calling function and avoid this kcalloc(). Then you can 
make this function return the total number of sg elements, which is 
actually at most original number of elements + 2, right? Then you can use 
that total number to allocate all new sg elements in one go to reduce the 
number of allocations.

> +	if (!splitters)
> +		return NULL;
> +
> +	nents = 0;

I would put these in initialisation:

+	int i, nents = 0;
+	size_t size = sizes[0], len;

etc.

> +	size = *sizes;
> +	curr = splitters;
> +	for_each_sg(in, sg, sg_nents(in), i) {
> +		if (skip > sg_dma_len(sg)) {
> +			skip -= sg_dma_len(sg);
> +			continue;
> +		}
> +		len = min_t(size_t, size, sg_dma_len(sg) - skip);
> +		if (!curr->in_sg0) {
> +			curr->in_sg0 = sg;
> +			curr->skip_sg0 = sg_dma_len(sg) - len;
>  		}
> -		if (xfer_len > 0) {
> -			(*new_sg_len)++;
> -			remain -= xfer_len;
> +		size -= len;
> +		nents++;
> +		if (!size) {
> +			curr->nents = nents;
> +			curr->len_last_sg = len;
> +			nents = 0;
> +			size = *(++sizes);
> +
> +			if (!--nb_splits)
> +				break;

This break won't be needed, because:

> +
> +			if (len < curr->len_last_sg) {

How is this possible? You just did

+			curr->len_last_sg = len;

> +				(splitters + 1)->in_sg0 = sg;

In general I like pointer arithmetics and use it always when I need a 
_pointer_, but in such cases I'd normally just write splitters[1].in_sg0, 
don't you think that would look better? Ditto everywhere below.

> +				(splitters + 1)->skip_sg0 = 0;
> +			}
> +			curr++;
>  		}
> -		if (remain <= 0)
> -			break;
>  	}
> -	WARN_ON(nfirst >= sglen);
>  
> -	sg0 = kmalloc_array(*new_sg_len, sizeof(struct scatterlist),
> -			    GFP_KERNEL);
> -	if (!sg0)
> -		return NULL;
> +	return splitters;
> +}
>  
> -	remain = size;
> -	for_each_sg(sg_first, sg, *new_sg_len, i) {
> -		dma_len = sg_dma_len(sg);
> -		sg0[i] = *sg;
> +static int sg_split(struct scatterlist *in, const int nb_splits,
> +		    const size_t *split_sizes, struct scatterlist **out,
> +		    gfp_t gfp_mask)
> +{
> +	int i, j;
> +	struct scatterlist *in_sg, *out_sg;
> +	struct sg_splitter *splitters, *split;
>  
> -		sg0[i].offset = nfirst_offset;
> -		nfirst_offset = 0;
> +	splitters = sg_calculate_split(in, 0, split_sizes, nb_splits, gfp_mask);
> +	if (!splitters)
> +		return -ENOMEM;
>  
> -		xfer_len = min_t(int, remain, dma_len - sg0[i].offset);
> -		xfer_len = roundup(xfer_len, 8);
> -		sg_dma_len(&sg0[i]) = xfer_len;
> +	for (i = 0; i < nb_splits; i++) {
> +		(splitters + i)->out_sg =
> +			kmalloc_array((splitters + i)->nents,
> +				      sizeof(struct scatterlist), gfp_mask);
> +		if (!(splitters + i)->out_sg)
> +			goto err;
> +	}
>  
> -		remain -= xfer_len;
> -		if (remain <= 0) {
> -			sg_mark_end(&sg0[i]);
> -			break;
> +	for (i = 0; i < nb_splits; i++) {

Maybe

+	for (i = 0, split = splitters; i < nb_splits; i++, split++) {

> +		split = splitters + i;
> +		in_sg = split->in_sg0;
> +		out_sg = split->out_sg;
> +		out[i] = out_sg;
> +		for (j = 0; j < split->nents; j++) {

+		for (j = 0; j < split->nents; j++, out_sg++) {

> +			out_sg[j] = *in_sg;

+			*out_sg = *in_sg;

> +			if (!j) {
> +				out_sg[j].offset = split->skip_sg0;
> +				sg_dma_len(&out_sg[j]) -= split->skip_sg0;

+				out_sg->offset = split->skip_sg0;
+				sg_dma_len(out_sg) -= split->skip_sg0;

etc.

> +			} else {
> +				out_sg[j].offset = 0;
> +			}
> +			in_sg = sg_next(in_sg);
>  		}
> +		sg_dma_len(out_sg + split->nents - 1) = split->len_last_sg;
> +		sg_mark_end(out_sg + split->nents - 1);
>  	}
>  
> -	return sg0;
> +	kfree(splitters);
> +	return 0;
> +
> +err:
> +	for (i = 0; i < nb_splits; i++)
> +		kfree((splitters + i)->out_sg);
> +	kfree(splitters);
> +	return -ENOMEM;
>  }
>  
>  static void pxa_camera_dma_irq(struct pxa_camera_dev *pcdev,
> @@ -391,14 +437,11 @@ static int pxa_init_dma_channel(struct pxa_camera_dev *pcdev,
>  				int cibr, int size, int offset)
>  {
>  	struct dma_chan *dma_chan = pcdev->dma_chans[channel];
> -	struct scatterlist *sg;
> +	struct scatterlist *sg = buf->sg[channel];
>  	int sglen;
>  	struct dma_async_tx_descriptor *tx;
>  
> -	sg = videobuf_sg_cut(dma->sglist, dma->sglen, offset, size, &sglen);
> -	if (!sg)
> -		goto fail;
> -
> +	sglen = sg_nents(sg);
>  	tx = dmaengine_prep_slave_sg(dma_chan, sg, sglen, DMA_DEV_TO_MEM,
>  				     DMA_PREP_INTERRUPT | DMA_CTRL_ACK);
>  	if (!tx) {
> @@ -421,7 +464,6 @@ static int pxa_init_dma_channel(struct pxa_camera_dev *pcdev,
>  	}
>  
>  	buf->descs[channel] = tx;
> -	buf->sg[channel] = sg;
>  	buf->sg_len[channel] = sglen;
>  	return 0;
>  fail:
> @@ -458,6 +500,7 @@ static int pxa_videobuf_prepare(struct videobuf_queue *vq,
>  	struct pxa_buffer *buf = container_of(vb, struct pxa_buffer, vb);
>  	int ret;
>  	int size_y, size_u = 0, size_v = 0;
> +	size_t sizes[3];
>  
>  	dev_dbg(dev, "%s (vb=0x%p) 0x%08lx %d\n", __func__,
>  		vb, vb->baddr, vb->bsize);
> @@ -513,6 +556,16 @@ static int pxa_videobuf_prepare(struct videobuf_queue *vq,
>  			size_y = size;
>  		}
>  
> +		sizes[0] = size_y;
> +		sizes[1] = size_u;
> +		sizes[2] = size_v;
> +		ret = sg_split(dma->sglist, pcdev->channels, sizes, buf->sg,
> +			       GFP_KERNEL);
> +		if (ret) {

In most places pxa_camera.c checks for (ret < 0), but no longer in all 
anyway.

Thanks
Guennadi

> +			dev_err(dev, "sg_split failed: %d\n", ret);
> +			goto fail;
> +		}
> +
>  		/* init DMA for Y channel */
>  		ret = pxa_init_dma_channel(pcdev, buf, dma, 0, CIBR0,
>  					   size_y, 0);
> 
--
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