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Date:   Sat, 26 Sep 2020 16:14:28 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Tomasz Figa <tfiga@chromium.org>
Cc:     Christoph Hellwig <hch@lst.de>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>,
        Joonyoung Shim <jy0922.shim@samsung.com>,
        Seung-Woo Kim <sw0312.kim@samsung.com>,
        Ben Skeggs <bskeggs@redhat.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Matt Porter <mporter@kernel.crashing.org>,
        iommu@lists.linux-foundation.org,
        Stefan Richter <stefanr@s5r6.in-berlin.de>,
        linux1394-devel@lists.sourceforge.net, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
        linux-parisc@vger.kernel.org, linux-samsung-soc@vger.kernel.org,
        nouveau@lists.freedesktop.org, netdev@vger.kernel.org,
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        alsa-devel@alsa-project.org
Subject: Re: [PATCH 17/18] dma-iommu: implement ->alloc_noncoherent
Message-ID: <20200926141428.GB10379@lst.de>
References: <20200915155122.1768241-1-hch@lst.de> <20200915155122.1768241-18-hch@lst.de> <20200925184622.GB3607091@chromium.org>
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On Fri, Sep 25, 2020 at 06:46:22PM +0000, Tomasz Figa wrote:
> > +static void *iommu_dma_alloc_noncoherent(struct device *dev, size_t size,
> > +		dma_addr_t *handle, enum dma_data_direction dir, gfp_t gfp)
> > +{
> > +	if (!gfpflags_allow_blocking(gfp)) {
> > +		struct page *page;
> > +
> > +		page = dma_common_alloc_pages(dev, size, handle, dir, gfp);
> > +		if (!page)
> > +			return NULL;
> > +		return page_address(page);
> > +	}
> > +
> > +	return iommu_dma_alloc_remap(dev, size, handle, gfp | __GFP_ZERO,
> > +				     PAGE_KERNEL, 0);
> 
> iommu_dma_alloc_remap() makes use of the DMA_ATTR_ALLOC_SINGLE_PAGES attribute
> to optimize the allocations for devices which don't care about how contiguous
> the backing memory is. Do you think we could add an attrs argument to this
> function and pass it there?
> 
> As ARM is being moved to the common iommu-dma layer as well, we'll probably
> make use of the argument to support the DMA_ATTR_NO_KERNEL_MAPPING attribute to
> conserve the vmalloc area.

We could probably at it.  However I wonder why this is something the
drivers should care about.  Isn't this really something that should
be a kernel-wide policy for a given system?