[PATCH] drm/mediatek: stop iterating dma addresses when sg_dma_len() == 0

[Chun-Kuang Hu]

Hi, Anand,

Anand K. Mistry <amistry at chromium.org> 於 2020年4月28日 週二 上午9:54寫道：
>
> On Sun, 26 Apr 2020 at 18:04, Chun-Kuang Hu <chunkuang.hu at kernel.org> wrote:
> >
> > Hi, Anand:
> >
> > Anand K Mistry <amistry at chromium.org> 於 2020年4月20日 週一 下午2:09寫道：
> > >
> > > If dma_map_sg() merges pages when creating the mapping, only the first
> > > entries will have a valid sg_dma_address() and sg_dma_len(), followed by
> > > entries with sg_dma_len() == 0.
> > >
> > > Signed-off-by: Anand K Mistry <amistry at google.com>
> > > ---
> > >  drivers/gpu/drm/mediatek/mtk_drm_gem.c | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/mediatek/mtk_drm_gem.c b/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> > > index b04a3c2b111e09..f8fd8b98c30e3d 100644
> > > --- a/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> > > +++ b/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> > > @@ -224,6 +224,9 @@ struct drm_gem_object *mtk_gem_prime_import_sg_table(struct drm_device *dev,
> > >
> > >         expected = sg_dma_address(sg->sgl);
> > >         for_each_sg(sg->sgl, s, sg->nents, i) {
> > > +               if (!sg_dma_len(s))
> > > +                       break;
> >
> > I think this should be 'continue'
>
> scatterlist.h has the comment:
> /*
>  * These macros should be used after a dma_map_sg call has been done
>  * to get bus addresses of each of the SG entries and their lengths.
>  * You should only work with the number of sg entries dma_map_sg
>  * returns, or alternatively stop on the first sg_dma_len(sg) which
>  * is 0.
>  */
>
> So breaking on the first sg_dma_len(sg) == 0 appears to be (one of)
> the documented approach.
>

Okay, you're right. So

Reviewed-by: Chun-Kuang Hu <chunkuang.hu at kernel.org>

> >
> > Regards,
> > Chun-Kuang.
> >
> > > +
> > >                 if (sg_dma_address(s) != expected) {
> > >                         DRM_ERROR("sg_table is not contiguous");
> > >                         ret = -EINVAL;
> > > --
> > > 2.26.1.301.g55bc3eb7cb9-goog
> > >
> > >
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