linux-kernel - Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Date:   Thu, 29 Nov 2018 19:35:06 +0100
From:   Christoph Hellwig <hch@....de>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, Christoph Hellwig <hch@....de>,
        Rob Clark <robdclark@...il.com>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

On Thu, Nov 29, 2018 at 09:24:17AM -0800, Tomasz Figa wrote:
> Whether the cache maintenance operation needs to actually do anything
> or not is a function of `dev`. We can have some devices that are
> coherent with CPU caches, and some that are not, on the same system.

Yes, but that part is not decided by these low-level helpers but their
callers in the DMA code (or maybe IOMMU code as well in the future).

> There is also the use case of using CMA with device-specific pools of
> memory reusable by the system when not used by the device and those
> would have to somehow get the pool to allocate from, but I wonder if
> struct device is the right way to pass such information. I'd see the
> pool given explicitly like cma_alloc(struct cma_pool *, size, flags)
> and perhaps a wrapper cma_alloc_default(size, flags) that is just a
> simple macro calling cma_alloc(&cma_pool_default, size, flags).

Yes, the cma APIs have quite a few warts that need addressing.
I have a few of those things on my todo list, but help is always
welcome.