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SubjectRe: [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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