Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

From: Christoph Hellwig <hch@lst.de>
To: Tomasz Figa <tfiga@chromium.org>
Cc: Vivek Gautam <vivek.gautam@codeaurora.org>,
	David Airlie <airlied@linux.ie>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Sean Paul <seanpaul@chromium.org>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	freedreno <freedreno@lists.freedesktop.org>,
	Christoph Hellwig <hch@lst.de>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*
Date: Thu, 29 Nov 2018 19:35:06 +0100	[thread overview]
Message-ID: <20181129183506.GC30281@lst.de> (raw)
In-Reply-To: <CAAFQd5A-Bh711VaNWhRJqcrofkBVCMFcNgqfztoyWJcUdhSu0Q@mail.gmail.com>

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.
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