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

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Date:   Tue, 17 Jul 2018 16:57:25 +0200
From:   Christoph Hellwig <hch@....de>
To:     Fredrik Noring <noring@...rew.org>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>, m.szyprowski@...sung.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        JuergenUrban@....de
Subject: Re: [PATCH] dma-mapping: Relax warnings for per-device areas

On Sun, Jul 15, 2018 at 02:28:27PM +0200, Fredrik Noring wrote:
> Hi Christoph, Robin,
> 
> On Thu, Jul 05, 2018 at 09:36:13PM +0200, 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.
> 
> Here are three other regressions related to the coherent mask WARN_ON_ONCE:

They are a pretty strong indication that yes, you should really set
the coherent mask if you ever do coherent allocations..