linux-kernel - Re: [PATCH 1/2] m68k: Set default dma mask for platform devices

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Date:   Sat, 19 May 2018 15:25:39 +1000 (AEST)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Christoph Hellwig <hch@....de>
cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, Greg Ungerer <gerg@...ux-m68k.org>
Subject: Re: [PATCH 1/2] m68k: Set default dma mask for platform devices

On Fri, 18 May 2018, Christoph Hellwig wrote:

> > This implementation of arch_setup_pdev_archdata() differs from the 
> > powerpc one, in that this one avoids clobbering a device dma mask 
> > which has already been initialized.
> 
> I think your implementation should move into the generic implementation 
> in drivers/base/platform.c instead of being stuck in m68k.
> 
> Also powerpc probably wants fixing, but that's something left to the
> ppc folks..

On powerpc, all platform devices get a dma mask. But they don't do that in 
drivers/base/platform.c, they use arch_setup_pdev_archdata(). Why didn't 
they take the approach you suggest?

How would I support the claim that replacing an empty platform device dma 
mask with 0xffffffff is safe on all architectures and platforms?

Is there no code conditional upon dev.coherent_dma_mask or dev.dma_mask 
that could misbehave? (Didn't I cite an example in the other thread?*)

If you can convince me that it is safe, I'd be happy to submit the patch 
you asked for.

For now, I still think that patching the platform driver was the correct 
patch*.

Maybe the real problem is your commit 205e1b7f51e4 ("dma-mapping: warn 
when there is no coherent_dma_mask"), because it assumes that all dma_ops 
implementations care about coherent_dma_mask.

The dma_map_ops implementations that do use coherent_dma_mask should 
simply fail when it is unset, right?

Would it not be better to revert your patch and fix the dma_map_ops 
failure paths, than to somehow audit all the platform drivers and patch 
drivers/base/platform.c?

Thanks.

* https://lkml.kernel.org/r/alpine.LNX.2.21.1805091804290.72%40nippy.intranet

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