Re: [PATCH v4 38/38] videobuf2: use sgtable-based scatterlist wrappers

[Marek Szyprowski]

Hi Michael,

On 12.05.2020 19:52, Ruhl, Michael J wrote:
>> -----Original Message-----
>> From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
>> Marek Szyprowski
>> Sent: Tuesday, May 12, 2020 5:01 AM
>> To: dri-devel@xxxxxxxxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx;
>> linaro-mm-sig@xxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>> Cc: Pawel Osciak <pawel@xxxxxxxxxx>; Bartlomiej Zolnierkiewicz
>> <b.zolnierkie@xxxxxxxxxxx>; David Airlie <airlied@xxxxxxxx>; linux-
>> media@xxxxxxxxxxxxxxx; Hans Verkuil <hverkuil-cisco@xxxxxxxxx>; Mauro
>> Carvalho Chehab <mchehab@xxxxxxxxxx>; Robin Murphy
>> <robin.murphy@xxxxxxx>; Christoph Hellwig <hch@xxxxxx>; linux-arm-
>> kernel@xxxxxxxxxxxxxxxxxxx; Marek Szyprowski
>> <m.szyprowski@xxxxxxxxxxx>
>> Subject: [PATCH v4 38/38] videobuf2: use sgtable-based scatterlist wrappers
>>
>> Use recently introduced common wrappers operating directly on the struct
>> sg_table objects and scatterlist page iterators to make the code a bit
>> more compact, robust, easier to follow and copy/paste safe.
>>
>> No functional change, because the code already properly did all the
>> scaterlist related calls.
>>
>> Signed-off-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
>> ---
>> For more information, see '[PATCH v4 00/38] DRM: fix struct sg_table nents
>> vs. orig_nents misuse' thread:
>> https://lore.kernel.org/dri-devel/20200512085710.14688-1-
>> m.szyprowski@xxxxxxxxxxx/T/
>> ---
>> .../media/common/videobuf2/videobuf2-dma-contig.c  | 41 ++++++++++----
>> --------
>> drivers/media/common/videobuf2/videobuf2-dma-sg.c  | 32 +++++++--------
>> --
>> drivers/media/common/videobuf2/videobuf2-vmalloc.c | 12 +++----
>> 3 files changed, 34 insertions(+), 51 deletions(-)
>>
>> diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
>> b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
>> index d3a3ee5..bf31a9d 100644
>> --- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
>> +++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
>> @@ -48,16 +48,15 @@ struct vb2_dc_buf {
>>
>> static unsigned long vb2_dc_get_contiguous_size(struct sg_table *sgt)
>> {
>> -	struct scatterlist *s;
>> 	dma_addr_t expected = sg_dma_address(sgt->sgl);
>> -	unsigned int i;
>> +	struct sg_dma_page_iter dma_iter;
>> 	unsigned long size = 0;
>>
>> -	for_each_sg(sgt->sgl, s, sgt->nents, i) {
>> -		if (sg_dma_address(s) != expected)
>> +	for_each_sgtable_dma_page(sgt, &dma_iter, 0) {
>> +		if (sg_page_iter_dma_address(&dma_iter) != expected)
>> 			break;
>> -		expected = sg_dma_address(s) + sg_dma_len(s);
>> -		size += sg_dma_len(s);
>> +		expected += PAGE_SIZE;
>> +		size += PAGE_SIZE;
> This code in drm_prime_t_contiguous_size and here.  I seem to remember seeing
> the same pattern in other drivers.
>
> Would it worthwhile to make this a helper as well?
I think I've identified such patterns in all DRM drivers and replaced 
with a common helper. So far I have no idea where to put such helper to 
make it available for media/videobuf2, so those a few lines are indeed 
duplicated here.
> Also, isn't the sg_dma_len() the actual length of the chunk we are looking at?
>
> If its I not PAGE_SIZE (ie. dma chunk is 4 * PAGE_SIZE?), does your loop/calculation still work?

scaterlist page iterators (for_each_sg_page/for_each_sg_dma_page and 
their sgtable variants) always operates on PAGE_SIZE units. They 
correctly handle larger sg_dma_len().


Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland