linux-kernel - Re: [PATCH v2 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Date:   Tue, 27 Nov 2018 23:39:40 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Vivek Gautam <vivek.gautam@...eaurora.org>
Cc:     airlied@...ux.ie, robdclark@...il.com, tfiga@...omium.org,
        linux-kernel@...r.kernel.org, freedreno@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v2 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

> +		/*
> +		 * dma_sync_sg_*() flush the physical pages, so point
> +		 * sg->dma_address to the physical ones for the right behavior.
> +		 */
> +		for_each_sg(msm_obj->sgt->sgl, s, msm_obj->sgt->nents, i)
> +			sg_dma_address(s) = sg_phys(s);
> +

I'm sorry, but this is completely bogus and not acceptable.

The only place that is allowed to initialize sg_dma_address is
dma_map_sg.  If the default dma ops don't work for your setup we have
major a problem and need to fix the dma api / iommu integration instead
of hacking around it.