linux-kernel - Re: [PATCH 31/33] dma-direct: reject too small dma masks

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Date:   Wed, 10 Jan 2018 16:32:26 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        linux-mips@...ux-mips.org, linux-ia64@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        Guan Xuetao <gxt@...c.pku.edu.cn>, linux-arch@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-c6x-dev@...ux-c6x.org,
        linux-hexagon@...r.kernel.org, x86@...nel.org,
        Konrad Rzeszutek Wilk <konrad@...nok.org>,
        linux-snps-arc@...ts.infradead.org,
        linux-m68k@...ts.linux-m68k.org, patches@...ups.riscv.org,
        linux-metag@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Michal Simek <monstr@...str.eu>, linux-parisc@...r.kernel.org,
        linux-cris-kernel@...s.com, linux-kernel@...r.kernel.org,
        linux-alpha@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 31/33] dma-direct: reject too small dma masks

On Wed, Jan 10, 2018 at 11:49:34AM +0000, Robin Murphy wrote:
>> +#ifdef CONFIG_ZONE_DMA
>> +	if (mask < DMA_BIT_MASK(ARCH_ZONE_DMA_BITS))
>> +		return 0;
>> +#else
>> +	/*
>> +	 * Because 32-bit DMA masks are so common we expect every architecture
>> +	 * to be able to satisfy them - either by not supporting more physical
>> +	 * memory, or by providing a ZONE_DMA32.  If neither is the case, the
>> +	 * architecture needs to use an IOMMU instead of the direct mapping.
>> +	 */
>> +	if (mask < DMA_BIT_MASK(32))
>> +		return 0;
>
> Do you think it's worth the effort to be a little more accommodating here? 
> i.e.:
>
> 		return dma_max_pfn(dev) >= max_pfn;
>
> We seem to have a fair few 28-31 bit masks for older hardware which 
> probably associates with host systems packing equivalently small amounts of 
> RAM.

And those devices don't have a ZONE_DMA?  I think we could do something
like that, but I'd rather have it as a separate commit with a good
explanation.  Maybe you can just send on on top of the series?