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Date:   Sat, 26 Sep 2020 17:25:27 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Matt Porter <mporter@...nel.crashing.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <iommu@...ts.linux-foundation.org>,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        linux1394-devel@...ts.sourceforge.net,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS <iommu@...ts.linux-foundation.org>, Joerg
        Roedel <joro@...tes.org>," <linux-arm-kernel@...ts.infradead.org>,
        linux-mips@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        nouveau@...ts.freedesktop.org, netdev@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-mm@...ck.org,
        alsa-devel@...a-project.org
Subject: Re: [PATCH 17/18] dma-iommu: implement ->alloc_noncoherent

On Sat, Sep 26, 2020 at 4:14 PM Christoph Hellwig <hch@....de> wrote:
>
> 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?

There are IOMMUs out there which support huge pages and those can
benefit *some* hardware depending on what kind of accesses they
perform, possibly on a per-buffer basis. At the same time, order > 0
allocations can be expensive, significantly affecting allocation
latency, so for devices which don't care about huge pages anyone would
prefer simple single-page allocations. Currently the drivers know the
best on whether the hardware they drive would care. There are some
decision factors listed in the documentation [1].

I can imagine cases where drivers could not be the best to decide
about this - for example, the workload could vary depending on the
userspace or a product decision regarding the performance vs
allocation latency, but we haven't seen such cases in practice yet.

[1] https://www.kernel.org/doc/html/latest/core-api/dma-attributes.html?highlight=dma_attr_alloc_single_pages#dma-attr-alloc-single-pages

Best regards,
Tomasz

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