linux-kernel - Re: [PATCH] dma/direct: turn ARCH_ZONE_DMA

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Message-ID: <6726a651c12d91ca22b9d8984745d90db5d507ec.camel@suse.de>
Date:   Thu, 31 Oct 2019 17:22:59 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Christoph Hellwig <hch@....de>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] dma/direct: turn ARCH_ZONE_DMA_BITS into a variable

On Thu, 2019-10-31 at 16:57 +0100, Christoph Hellwig wrote:
> On Thu, Oct 31, 2019 at 04:53:13PM +0100, Nicolas Saenz Julienne wrote:
> > > > +#define ARM64_ZONE_DMA_BITS	30
> > > > +
> > > >  /*
> > > >   * We need to be able to catch inadvertent references to memstart_addr
> > > >   * that occur (potentially in generic code) before
> > > > arm64_memblock_init()
> > > > @@ -424,6 +427,8 @@ void __init arm64_memblock_init(void)
> > > >  	else
> > > >  		arm64_dma_phys_limit = PHYS_MASK + 1;
> > > >  
> > > > +	zone_dma_bits = ARM64_ZONE_DMA_BITS;
> > > > +
> > > >  	reserve_crashkernel();
> > > 
> > > This actually adds a new limit, as there wasn't one before for arm64.
> > 
> > Well, as zone_dma_bits is only relevant in dma/direct when ZONE_DMA is
> > defined
> > I figured it doesn't matter if the variable is set conditionally to ZONE_DMA
> > or
> > not.
> 
> I'd much prefer that to do separately.

OK, I see what you mean now. It's wrong indeed.

The trouble is the ZONE_DMA series[1] in arm64, also due for v5.5, will be
affected by this patch. I don't know the right way to approach this problem
since depending on the merge order, this patch should be updated or the arm64
ZONE_DMA series fixed.

Maybe it's easier to just wait for v5.6.

Regards,
Nicolas

[1] https://lkml.org/lkml/2019/9/11/734


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