linux-kernel - Re: [RFC PATCH 1/4] dma-mapping: Generalise dma_32bit

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Date:   Wed, 11 Jul 2018 17:56:40 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     m.szyprowski@...sung.com, iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-acpi@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        lorenzo.pieralisi@....com, hanjun.guo@...aro.org,
        sudeep.holla@....com, robh+dt@...nel.org, frowand.list@...il.com,
        gregkh@...uxfoundation.org, joro@...tes.org, x86@...nel.org
Subject: Re: [RFC PATCH 1/4] dma-mapping: Generalise dma_32bit_limit flag

On 10/07/18 19:04, Christoph Hellwig wrote:
> On Tue, Jul 10, 2018 at 06:17:16PM +0100, Robin Murphy wrote:
>> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
>> index 8be8106270c2..95e185347e34 100644
>> --- a/kernel/dma/direct.c
>> +++ b/kernel/dma/direct.c
>> @@ -183,7 +183,7 @@ int dma_direct_supported(struct device *dev, u64 mask)
>>   	 * Various PCI/PCIe bridges have broken support for > 32bit DMA even
>>   	 * if the device itself might support it.
>>   	 */
>> -	if (dev->dma_32bit_limit && mask > DMA_BIT_MASK(32))
>> +	if (dev->bus_dma_mask && mask > dev->bus_dma_mask)
>>   		return 0;
> 
> The comment above this check needs an updated (or just be removed).

Right, I'll give it a tweak. I could also do with actually getting the 
field name correct in via_no_dac_cb()...

> Also we still have a few architectures not using dma-direct. I guess
> most were doing fine without such limits anyway, but at least arm
> will probably need an equivalent check.

Indeed, once we've found an approach that everyone's happy with we can 
have a more thorough audit of exactly where else it needs to be applied. 
FWIW I'm not aware of any 32-bit Arm systems affected by this*, but if 
they do exist then at least there's no risk of regression since they've 
always been busted.

Robin.


* Not counting the somewhat-similar StrongArm DMA controller bug where 
one bit in the *middle* of the mask is unusable. Let's keep that 
confined to the Arm dmabounce code and never speak of it...