netdev - Re: [PATCH 05/28] media/v4l2: remove V4L2-FLAG-MEMORY-NON-CONSISTENT

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Message-ID: <20200820050214.GA4815@lst.de>
Date:   Thu, 20 Aug 2020 07:02:14 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Tomasz Figa <tfiga@...omium.org>, Christoph Hellwig <hch@....de>,
        alsa-devel@...a-project.org, linux-ia64@...r.kernel.org,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        nouveau@...ts.freedesktop.org, linux-nvme@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        linux-mm@...ck.org, Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        linux-scsi@...r.kernel.org,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Matt Porter <mporter@...nel.crashing.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Pawel Osciak <pawel@...iak.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-parisc@...r.kernel.org, netdev@...r.kernel.org,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH 05/28] media/v4l2: remove
 V4L2-FLAG-MEMORY-NON-CONSISTENT

On Wed, Aug 19, 2020 at 03:07:04PM +0100, Robin Murphy wrote:
>> FWIW, I asked back in time what the plan is for non-coherent
>> allocations and it seemed like DMA_ATTR_NON_CONSISTENT and
>> dma_sync_*() was supposed to be the right thing to go with. [2] The
>> same thread also explains why dma_alloc_pages() isn't suitable for the
>> users of dma_alloc_attrs() and DMA_ATTR_NON_CONSISTENT.
>
> AFAICS even back then Christoph was implying getting rid of NON_CONSISTENT 
> and *replacing* it with something streaming-API-based - i.e. this series - 
> not encouraging mixing the existing APIs. It doesn't seem impossible to 
> implement a remapping version of this new dma_alloc_pages() for 
> IOMMU-backed ops if it's really warranted (although at that point it seems 
> like "non-coherent" vb2-dc starts to have significant conceptual overlap 
> with vb2-sg).

You can alway vmap the returned pages from dma_alloc_pages, but it will
make cache invalidation hell - you'll need to use
invalidate_kernel_vmap_range and flush_kernel_vmap_range to properly
handle virtually indexed caches.

Or with remapping you mean using the iommu do de-scatter/gather?

You can implement that trivially implement it yourself for the iommu
case:

{
	merge_boundary = dma_get_merge_boundary(dev);
	if (!merge_boundary || merge_boundary > chunk_size - 1) {
		/* can't coalesce */
		return -EINVAL;
	}

	
	nents = DIV_ROUND_UP(total_size, chunk_size);
	sg = sgl_alloc();
	for_each_sgl() {
		sg->page = __alloc_pages(get_order(chunk_size))
		sg->len = chunk_size;
	}
	dma_map_sg(sg, DMA_ATTR_SKIP_CPU_SYNC);
	// you are guaranteed to get a single dma_addr out
}

Of course this still uses the scatterlist structure with its annoying
mix of input and output parametes, so I'd rather not expose it as
an official API at the DMA layer.