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

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Date:   Tue, 12 May 2020 22:33:54 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     "Ruhl, Michael J" <michael.j.ruhl@...el.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     Pawel Osciak <pawel@...iak.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        David Airlie <airlied@...ux.ie>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 38/38] videobuf2: use sgtable-based scatterlist
 wrappers

Hi Michael,

On 12.05.2020 19:52, Ruhl, Michael J wrote:
>> -----Original Message-----
>> From: dri-devel <dri-devel-bounces@...ts.freedesktop.org> On Behalf Of
>> Marek Szyprowski
>> Sent: Tuesday, May 12, 2020 5:01 AM
>> To: dri-devel@...ts.freedesktop.org; iommu@...ts.linux-foundation.org;
>> linaro-mm-sig@...ts.linaro.org; linux-kernel@...r.kernel.org
>> Cc: Pawel Osciak <pawel@...iak.com>; Bartlomiej Zolnierkiewicz
>> <b.zolnierkie@...sung.com>; David Airlie <airlied@...ux.ie>; linux-
>> media@...r.kernel.org; Hans Verkuil <hverkuil-cisco@...all.nl>; Mauro
>> Carvalho Chehab <mchehab@...nel.org>; Robin Murphy
>> <robin.murphy@....com>; Christoph Hellwig <hch@....de>; linux-arm-
>> kernel@...ts.infradead.org; Marek Szyprowski
>> <m.szyprowski@...sung.com>
>> 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@...sung.com>
>> ---
>> 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@...sung.com/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