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Subject: Re: [v4,1/3] drm/prime: use dma length macro when mapping sg
From:   Marek Szyprowski <m.szyprowski@samsung.com>
To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Shane Francis <bigbeeshane@gmail.com>,
        dri-devel@lists.freedesktop.org
Cc:     airlied@linux.ie, linux-kernel@vger.kernel.org,
        amd-gfx-request@lists.freedesktop.org, alexander.deucher@amd.com
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Date:   Mon, 30 Mar 2020 10:18:21 +0200
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Hi

On 2020-03-27 10:10, Marek Szyprowski wrote:
> Hi Christian,
>
> On 2020-03-27 09:11, Christian König wrote:
>> Am 27.03.20 um 08:54 schrieb Marek Szyprowski:
>>> On 2020-03-25 10:07, Shane Francis wrote:
>>>> As dma_map_sg can reorganize scatter-gather lists in a
>>>> way that can cause some later segments to be empty we should
>>>> always use the sg_dma_len macro to fetch the actual length.
>>>>
>>>> This could now be 0 and not need to be mapped to a page or
>>>> address array
>>>>
>>>> Signed-off-by: Shane Francis <bigbeeshane@gmail.com>
>>>> Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
>>> This patch landed in linux-next 20200326 and it causes a kernel 
>>> panic on
>>> various Exynos SoC based boards.
>>>> ---
>>>>    drivers/gpu/drm/drm_prime.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>>>> index 86d9b0e45c8c..1de2cde2277c 100644
>>>> --- a/drivers/gpu/drm/drm_prime.c
>>>> +++ b/drivers/gpu/drm/drm_prime.c
>>>> @@ -967,7 +967,7 @@ int drm_prime_sg_to_page_addr_arrays(struct 
>>>> sg_table *sgt, struct page **pages,
>>>>           index = 0;
>>>>        for_each_sg(sgt->sgl, sg, sgt->nents, count) {
>>>> -        len = sg->length;
>>>> +        len = sg_dma_len(sg);
>>>>            page = sg_page(sg);
>>>>            addr = sg_dma_address(sg);
>>> Sorry, but this code is wrong :(
>>
>> Well it is at least better than before because it makes most drivers 
>> work correctly again.
>
> Well, I'm not sure that a half-broken fix should be considered as a 
> fix ;)
>
> Anyway, I just got the comment from Shane, that my patch is fixing the 
> issues with amdgpu and radeon, while still working fine for exynos, so 
> it is indeed a proper fix.

Today I've noticed that this patch went to final v5.6 without even a day 
of testing in linux-next, so v5.6 is broken on Exynos and probably a few 
other ARM archs, which rely on the drm_prime_sg_to_page_addr_arrays 
function.

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