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

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Date:   Thu, 5 Jul 2018 21:36:13 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     hch@....de, m.szyprowski@...sung.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        noring@...rew.org, JuergenUrban@....de
Subject: Re: [PATCH] dma-mapping: Relax warnings for per-device areas

> -	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.