Re: [PATCH v2 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

[Vivek Gautam]

Hi Tomasz,

On Wed, Nov 28, 2018 at 8:39 AM Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
>
> Hi Vivek,
>
> On Tue, Nov 27, 2018 at 6:37 AM Vivek Gautam
> <vivek.gautam@xxxxxxxxxxxxxx> wrote:
> >
> > dma_map_sg() expects a DMA domain. However, the drm devices
> > have been traditionally using unmanaged iommu domain which
> > is non-dma type. Using dma mapping APIs with that domain is bad.
> >
> > Replace dma_map_sg() calls with dma_sync_sg_for_device{|cpu}()
> > to do the cache maintenance.
> >
> > Signed-off-by: Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx>
> > Suggested-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > Cc: Rob Clark <robdclark@xxxxxxxxx>
> > Cc: Jordan Crouse <jcrouse@xxxxxxxxxxxxxx>
> > Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> > ---
> >
> > Changes since v1:
> >  - Addressed Jordan's and Tomasz's comments for
> >    - moving sg dma addresses preparation out of the coditional
> >      check to the main path so we do it irrespective of whether
> >      the buffer is cached or uncached.
> >    - Enhance the comment to explain this dma addresses prepartion.
> >
>
> Thanks for the patch. Some comments inline.
>
> >  drivers/gpu/drm/msm/msm_gem.c | 31 ++++++++++++++++++++++---------
> >  1 file changed, 22 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> > index 00c795ced02c..1811ac23a31c 100644
> > --- a/drivers/gpu/drm/msm/msm_gem.c
> > +++ b/drivers/gpu/drm/msm/msm_gem.c
> > @@ -81,6 +81,8 @@ static struct page **get_pages(struct drm_gem_object *obj)
> >                 struct drm_device *dev = obj->dev;
> >                 struct page **p;
> >                 int npages = obj->size >> PAGE_SHIFT;
> > +               struct scatterlist *s;
> > +               int i;
> >
> >                 if (use_pages(obj))
> >                         p = drm_gem_get_pages(obj);
> > @@ -104,12 +106,21 @@ static struct page **get_pages(struct drm_gem_object *obj)
> >                         return ptr;
> >                 }
> >
> > -               /* For non-cached buffers, ensure the new pages are clean
> > +               /*
> > +                * dma_sync_sg_*() flush the physical pages, so point
> > +                * sg->dma_address to the physical ones for the right behavior.
>
> The two halves of the sequence don't really relate to each other. An
> sglist has the `page` field for the purpose of pointing to physical
> pages. The `dma_address` field is for DMA addresses, which are not
> equivalent to physical addresses. I'd rewrite it like this;

I guess I was lenient in using the physical pages and dma address words.

>
> /*
>  * Some implementations of the DMA mapping ops expect
>  * physical addresses of the pages to be stored as DMA
>  * addresses of the sglist entries. To work around it,
>  * set them here explicitly.
>  */

Will update as suggested.

> > +                */
> > +               for_each_sg(msm_obj->sgt->sgl, s, msm_obj->sgt->nents, i)
> > +                       sg_dma_address(s) = sg_phys(s);
> > +
> > +               /*
> > +                * For non-cached buffers, ensure the new pages are clean
> >                  * because display controller, GPU, etc. are not coherent:
> >                  */
> > -               if (msm_obj->flags & (MSM_BO_WC|MSM_BO_UNCACHED))
> > -                       dma_map_sg(dev->dev, msm_obj->sgt->sgl,
> > -                                       msm_obj->sgt->nents, DMA_BIDIRECTIONAL);
> > +               if (msm_obj->flags & (MSM_BO_WC | MSM_BO_UNCACHED))
> > +                       dma_sync_sg_for_device(dev->dev, msm_obj->sgt->sgl,
> > +                                              msm_obj->sgt->nents,
> > +                                              DMA_TO_DEVICE);
>
> Why changing from DMA_BIDIRECTIONAL?

Yea, I went back and checked that we wanted to do this for both.
Will keep DMA_BIDIRECTIONAL intact.

>
> >         }
> >
> >         return msm_obj->pages;
> > @@ -133,14 +144,16 @@ static void put_pages(struct drm_gem_object *obj)
> >
> >         if (msm_obj->pages) {
> >                 if (msm_obj->sgt) {
> > -                       /* For non-cached buffers, ensure the new
> > +                       /*
> > +                        * For non-cached buffers, ensure the new
> >                          * pages are clean because display controller,
> >                          * GPU, etc. are not coherent:
> >                          */
> > -                       if (msm_obj->flags & (MSM_BO_WC|MSM_BO_UNCACHED))
> > -                               dma_unmap_sg(obj->dev->dev, msm_obj->sgt->sgl,
> > -                                            msm_obj->sgt->nents,
> > -                                            DMA_BIDIRECTIONAL);
> > +                       if (msm_obj->flags & (MSM_BO_WC | MSM_BO_UNCACHED))
> > +                               dma_sync_sg_for_cpu(obj->dev->dev,
> > +                                                   msm_obj->sgt->sgl,
> > +                                                   msm_obj->sgt->nents,
> > +                                                   DMA_FROM_DEVICE);
>
> Ditto.

Sure, will change this as well.

Thanks & Regards
Vivek

>
> Best regards,
> Tomasz



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