linux-kernel - Re: [PATCH 5/8] arm64: use ZONE_DMA on DMA addressing limited devices

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Date:   Thu, 01 Aug 2019 17:44:09 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     phill@...pberryi.org, devicetree@...r.kernel.org,
        linux-rpi-kernel@...ts.infradead.org, f.fainelli@...il.com,
        frowand.list@...il.com, eric@...olt.net, marc.zyngier@....com,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        robh+dt@...nel.org, wahrenst@....net, mbrugger@...e.com,
        akpm@...ux-foundation.org, Robin Murphy <robin.murphy@....com>,
        hch@....de, linux-arm-kernel@...ts.infradead.org,
        m.szyprowski@...sung.com
Subject: Re: [PATCH 5/8] arm64: use ZONE_DMA on DMA addressing limited
 devices

On Wed, 2019-07-31 at 18:07 +0100, Catalin Marinas wrote:
> On Wed, Jul 31, 2019 at 05:47:48PM +0200, Nicolas Saenz Julienne wrote:
> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > index 1c4ffabbe1cb..f5279ef85756 100644
> > --- a/arch/arm64/mm/init.c
> > +++ b/arch/arm64/mm/init.c
> > @@ -50,6 +50,13 @@
> >  s64 memstart_addr __ro_after_init = -1;
> >  EXPORT_SYMBOL(memstart_addr);
> >  
> > +/*
> > + * We might create both a ZONE_DMA and ZONE_DMA32. ZONE_DMA is needed if
> > there
> > + * are periferals unable to address the first naturally aligned 4GB of ram.
> > + * ZONE_DMA32 will be expanded to cover the rest of that memory. If such
> > + * limitations doesn't exist only ZONE_DMA32 is created.
> > + */
> 
> Shouldn't we instead only create ZONE_DMA to cover the whole 32-bit
> range and leave ZONE_DMA32 empty? Can__GFP_DMA allocations fall back
> onto ZONE_DMA32?

Hi Catalin, thanks for the review.

You're right, the GFP_DMA page allocation will fail with a nasty dmesg error if
ZONE_DMA is configured but empty. Unsurprisingly the opposite situation is fine
(GFP_DMA32 with an empty ZONE_DMA32).

I switched to the scheme you're suggesting for the next version of the series.
The comment will be something the likes of this:

/*
 * We create both a ZONE_DMA and ZONE_DMA32. ZONE_DMA's size is decided based
 * on whether the SoC's peripherals are able to address the first naturally
 * aligned 4 GB of ram.
 *
 * If limited, ZONE_DMA covers that area and ZONE_DMA32 the rest of that 32 bit
 * addressable memory.
 *
 * If not ZONE_DMA is expanded to cover the whole 32 bit addressable memory and
 * ZONE_DMA32 is left empty.
 */

 Regards,
 Nicolas



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