Subject: Re: [PATCH 31/33] dma-direct: reject too small dma masks
To: Christoph Hellwig <hch@lst.de>
Cc: iommu@lists.linux-foundation.org, linux-mips@linux-mips.org,
        linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
        sparclinux@vger.kernel.org, Guan Xuetao <gxt@mprc.pku.edu.cn>,
        linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
        linux-c6x-dev@linux-c6x.org, linux-hexagon@vger.kernel.org,
        x86@kernel.org, Konrad Rzeszutek Wilk <konrad@darnok.org>,
        linux-snps-arc@lists.infradead.org,
        linux-m68k@lists.linux-m68k.org, patches@groups.riscv.org,
        linux-metag@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        Michal Simek <monstr@monstr.eu>, linux-parisc@vger.kernel.org,
        linux-cris-kernel@axis.com, linux-kernel@vger.kernel.org,
        linux-alpha@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
References: <20180110080027.13879-1-hch@lst.de>
 <20180110080027.13879-32-hch@lst.de>
 <0bcca030-a8da-c34a-a905-707986689f33@arm.com>
 <20180110153226.GE17790@lst.de>
From: Robin Murphy <robin.murphy@arm.com>
Message-ID: <619b174d-f38d-8d9e-dfd2-cc3a64ace446@arm.com>
Date: Wed, 10 Jan 2018 17:00:30 +0000
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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.