linux-kernel - Re: [PATCH] dma-mapping: Relax warnings for per-device areas

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Date:   Fri, 6 Jul 2018 12:57:11 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     noring@...rew.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, JuergenUrban@....de
Subject: Re: [PATCH] dma-mapping: Relax warnings for per-device areas

On 05/07/18 20:36, Christoph Hellwig wrote:
>> -	BUG_ON(!ops);
>> -	WARN_ON_ONCE(dev && !dev->coherent_dma_mask);
>> -
>>   	if (dma_alloc_from_dev_coherent(dev, size, dma_handle, &cpu_addr))
>>   		return cpu_addr;
>>   
>> +	BUG_ON(!ops);
>> +	WARN_ON_ONCE(dev && !dev->coherent_dma_mask);
> 
> I think doing dma on a device without ops is completely broken no matter
> what you think of it, so I very much disagree with that part of the change.
> 
> Also while I don't think not having a dma mask is a good idea even for
> a driver purely using dma coherent pools.  If the pools really are on
> the device itself I can see why it might not matter, but for the case
> commonly used on some ARM SOCs where we just reserve memory for certain
> devices from a system pool it very much does matter.
> 
> There really is no good excuse to not set a coherent mask in the drivers.

Right, I was rather on the fence about this - on the one hand it is 
objectively wrong per the API for drivers to call dma_alloc_coherent() 
without a prior successful dma_set_coherent_mask() call, but then I 
thought that in the case when they're *only* using it as a proxy for 
dma_alloc_from_dev_coherent() and explicitly don't want regular 
allocations from kernel memory to ever happen, then maybe it might be 
somewhat reasonable. But indeed I hadn't really given enough thought to 
the reserved-memory carveout case, where we definitely don't want to let 
a legitimate warning be hidden on a developer's machine but hit by users 
with different system configurations.

Fredrik, are you happy to fix up your driver to initialise a suitable 
mask at probe time?

Robin.