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    SubjectRe: [PATCH 5/8] arm64: use ZONE_DMA on DMA addressing limited devices
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    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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