linux-kernel - Re: [PATCH v5 3/4] arm64: use both ZONE_DMA and ZONE

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Message-ID: <20190911143527.GB43864@C02TF0J2HF1T.local>
Date:   Wed, 11 Sep 2019 15:35:27 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     hch@....de, wahrenst@....net, marc.zyngier@....com,
        robh+dt@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
        Will Deacon <will@...nel.org>, f.fainelli@...il.com,
        robin.murphy@....com, linux-kernel@...r.kernel.org,
        mbrugger@...e.com, linux-rpi-kernel@...ts.infradead.org,
        phill@...pberrypi.org, m.szyprowski@...sung.com
Subject: Re: [PATCH v5 3/4] arm64: use both ZONE_DMA and ZONE_DMA32

On Wed, Sep 11, 2019 at 12:54:38PM +0200, Nicolas Saenz Julienne wrote:
> On Mon, 2019-09-09 at 11:58 +0200, Nicolas Saenz Julienne wrote:
> >  /*
> > - * Return the maximum physical address for ZONE_DMA32 (DMA_BIT_MASK(32)). It
> > - * currently assumes that for memory starting above 4G, 32-bit devices will
> > - * use a DMA offset.
> > + * Return the maximum physical address for a zone with a given address size
> > + * limit. It currently assumes that for memory starting above 4G, 32-bit
> > + * devices will use a DMA offset.
> >   */
> > -static phys_addr_t __init max_zone_dma32_phys(void)
> > +static phys_addr_t __init max_zone_phys(unsigned int zone_bits)
> >  {
> >         phys_addr_t offset = memblock_start_of_DRAM() & GENMASK_ULL(63, 32);
> > -       return min(offset + (1ULL << 32), memblock_end_of_DRAM());
> > +       return min(offset + (1ULL << zone_bits), memblock_end_of_DRAM());
> >  }
> 
> while testing other code on top of this series on odd arm64 machines I found an
> issue: when memblock_start_of_DRAM() != 0, max_zone_phys() isn't taking into
> account the offset to the beginning of memory. This doesn't matter with
> zone_bits == 32 but it does when zone_bits == 30.

I thought about this but I confused myself and the only case I had in
mind was an AMD Seattle system with RAM starting at 4GB.

What we need from this function is that the lowest naturally aligned
2^30 RAM is covered by ZONE_DMA while the rest to 2^32 are ZONE_DMA32.
This assumed that devices only capable of 30-bit (or 32-bit), have the
top address bits hardwired to be able access the bottom of the memory
(and this would be expressed in DT as the DMA offset).

I guess the fix here is to use GENMASK_ULL(63, zone_bits).

-- 
Catalin