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Date: Wed, 13 May 2020 12:01:37 +0000
From: "Ruhl, Michael J" <michael.j.ruhl@...el.com>
To: Marek Szyprowski <m.szyprowski@...sung.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
>-----Original Message-----
>From: Marek Szyprowski <m.szyprowski@...sung.com>
>Sent: Tuesday, May 12, 2020 4:34 PM
>To: Ruhl, Michael J <michael.j.ruhl@...el.com>; 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
>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.
I was thinking of drivers outside of DRM/media. Specifically RDMA.
However, looking at that code, I see that my memory was a little off.
It is working with continuous pages, but not finding the size.
>> 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().
Ahh, ok, I see.
Thank you!
Mike
>
>Best regards
>--
>Marek Szyprowski, PhD
>Samsung R&D Institute Poland
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