linux-kernel - Re: [PATCH 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Date:   Tue, 20 Nov 2018 08:41:48 -0700
From:   Jordan Crouse <jcrouse@...eaurora.org>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     robdclark@...il.com, airlied@...ux.ie,
        linux-kernel@...r.kernel.org, freedreno@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        tfiga@...omium.org
Subject: Re: [PATCH 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

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@...eaurora.org>
> Suggested-by: Tomasz Figa <tfiga@...omium.org>
> ---
> 
> 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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