Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

From: Oleksandr Andrushchenko <andr2000@gmail.com>
To: Christoph Hellwig <hch@infradead.org>, Daniel Vetter <daniel@ffwll.ch>
Cc: Juergen Gross <jgross@suse.com>,
	Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Daniel Vetter <daniel.vetter@intel.com>,
	xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com
Subject: Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent
Date: Fri, 21 Dec 2018 11:16:44 +0200	[thread overview]
Message-ID: <83a4be07-a06f-5585-66f6-4973f80dbfba@gmail.com> (raw)
In-Reply-To: <20181220183851.GA27025@infradead.org>

On 12/20/18 8:38 PM, Christoph Hellwig wrote:
> On Thu, Dec 20, 2018 at 07:35:15PM +0100, Daniel Vetter wrote:
>>> Err, with streaming DMA buffer sharing is trivial.  The coherent DMA
>>> allocator is what causes all kinds of horrible hacks that can't actually
>>> work on various platforms.
>> Hm, I thought the streaming dma api is the one that causes bounce
>> buffers and all that fun. If you're unlucky at least.
> Yes it may.  But even if that happens everything will actually work,
> just slower.  While the dma coherent API is simply broken.
>
> But if you don't want bounce buffering you need to use the dma
> noncoherent allocator as proposed here:
>
> 	https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031982.html
>
> which combines allocating memory that doesn't need to be bounce
> buffered with a sharing scheme that can actually work.
So, the bottom line will be: I can use DMA API for what I need, but:
1. I need to remove GFP_USER
2. No need for DMA32 (so no chance for bouncing to step in)
3. I may need to check if mapping and unmapping of the buffer
at once will also help, e.g. no need to have the buffer mapped until
it is destroyed
Did I get it all right?

Thank you,
Oleksandr