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

[Tomasz Figa <tfiga@chromium.org>]

Hi Vivek,

On Tue, Nov 27, 2018 at 6:37 AM Vivek Gautam
<vivek.gautam@codeaurora.org> 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@codeaurora.org>
> Suggested-by: Tomasz Figa <tfiga@chromium.org>
> Cc: Rob Clark <robdclark@gmail.com>
> Cc: Jordan Crouse <jcrouse@codeaurora.org>
> Cc: Sean Paul <seanpaul@chromium.org>
> ---
>
> 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;

/*
 * 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.
 */

> +                */
> +               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?

>         }
>
>         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.

Best regards,
Tomasz