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

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Message-ID: <619b174d-f38d-8d9e-dfd2-cc3a64ace446@arm.com>
Date:   Wed, 10 Jan 2018 17:00:30 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     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 10/01/18 15:32, Christoph Hellwig wrote:
> 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?

Good point - other than the IXP4xx platform and possibly the Broadcom 
network drivers, it's probably only x86-relevant stuff where the concern 
is moot. Let's just keep the simple assumption then, until actually 
proven otherwise.

Robin.