On Tue, Nov 20, 2018 at 03:24:37PM +0530, Vivek Gautam 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>
> ---
>
> Tested on an MTP sdm845:
> https://github.com/vivekgautam1/linux/tree/v4.19/sdm845-mtp-display-working
>
> drivers/gpu/drm/msm/msm_gem.c | 27 ++++++++++++++++++++-------
> 1 file changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> index 00c795ced02c..d7a7af610803 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);
> @@ -107,9 +109,19 @@ static struct page **get_pages(struct drm_gem_object *obj)
> /* 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)) {
> + /*
> + * Fake up the SG table so that dma_sync_sg_*()
> + * can be used to flush the pages associated with it.
> + */
We aren't really faking. The table is real, we are just slightly abusing the
sg_dma_address() which makes this comment a bit misleading. Instead I would
probably say something like:
/* dma_sync_sg_* flushes pages using sg_dma_address() so point it at the
* physical page for the right behavior */
Or something like that.
> + for_each_sg(msm_obj->sgt->sgl, s,
> + msm_obj->sgt->nents, i)
> + sg_dma_address(s) = sg_phys(s);
> +
I'm wondering - wouldn't we want to do this association for cached buffers to so
we could sync them correctly in cpu_prep and cpu_fini? Maybe it wouldn't hurt
to put this association in the main path (obviously the sync should stay inside
the conditional for uncached buffers).
> + dma_sync_sg_for_device(dev->dev, msm_obj->sgt->sgl,
> + msm_obj->sgt->nents,
> + DMA_TO_DEVICE);
> + }
> }
>
> return msm_obj->pages;
> @@ -137,10 +149,11 @@ static void put_pages(struct drm_gem_object *obj)
> * 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);
>
> sg_free_table(msm_obj->sgt);
> kfree(msm_obj->sgt);
--
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