linux-kernel - Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal memory size

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Message-ID: <7a34c74f-7c83-84bb-441b-a729adce2beb@arm.com>
Date:   Thu, 27 Sep 2018 16:07:50 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 5/5] dma-direct: always allow dma mask <= physiscal memory
 size

[ oops, I should have looked at the replies first, now I see Ben already 
had the same thing to say about #3... ]

On 27/09/18 14:49, Christoph Hellwig wrote:
> On Thu, Sep 27, 2018 at 11:50:14AM +1000, Benjamin Herrenschmidt wrote:
>>> -	 * 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 < phys_to_dma(dev, DMA_BIT_MASK(32)))
>>> +	u64 min_mask;
>>> +
>>> +	if (IS_ENABLED(CONFIG_ZONE_DMA))
>>> +		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
>>> +	else
>>> +		min_mask = min_t(u64, DMA_BIT_MASK(32),
>>> +				 (max_pfn - 1) << PAGE_SHIFT);
>>> +
>>> +	if (mask >= phys_to_dma(dev, min_mask))
>>>   		return 0;
>>
>> nitpick ... to be completely "correct", I would have written
>>
>> 	if (IS_ENABLED(CONFIG_ZONE_DMA))
>> 		min_mask = DMA_BIT_MASK(ARCH_ZONE_DMA_BITS);
>> 	else
>> 		min_mask = DMA_BIT_MASK(32);
>>
>> 	min_mask = min_t(u64, min_mask,	(max_pfn - 1) << PAGE_SHIFT);
>>
>> In "theory" it's also ok to have a mask < ZONE_DMA_BITS as long as it's
>> big enough to fit all memory :-)
> 
> Yeah, we could do that.

FWIW I like it even if just for looking slightly more readable. With 
that fixup,

Reviewed-by: Robin Murphy <robin.murphy@....com>