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From: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
Subject: Re: [RFC, v3 9/9] media: platform: Add Mediatek ISP P1 shared memory
	device
Date: Fri, 26 Jul 2019 12:04:23 +0100
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To: Tomasz Figa <tfiga-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>, Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
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List-Id: devicetree@vger.kernel.org

On 26/07/2019 08:42, Tomasz Figa wrote:
> On Fri, Jul 26, 2019 at 4:41 PM Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org> wrote:
>>
>> On Fri, Jul 26, 2019 at 02:15:14PM +0900, Tomasz Figa wrote:
>>> Could you try dma_get_sgtable() with the SCP struct device and then
>>> dma_map_sg() with the P1 struct device?
>>
>> Please don't do that.  dma_get_sgtable is a pretty broken API (see
>> the common near the arm implementation) and we should not add more
>> users of it.  If you want a piece of memory that can be mapped to
>> multiple devices allocate it using alloc_pages and then just map
>> it to each device.
> 
> Thanks for taking a look at this thread.
> 
> Unfortunately that wouldn't work. We have a specific reserved memory
> pool that is the only memory area accessible to one of the devices.
> Any idea how to handle this?

If it's reserved in the sense of being outside struct-page-backed 
"kernel memory", then provided you have a consistent CPU physical 
address it might be reasonable for other devices to access it via 
dma_map_resource().

Robin.