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Date:   Tue, 28 Jun 2022 23:25:42 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Michael Schmitz <schmitzmic@gmail.com>
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Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS
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On Wed, Jun 29, 2022 at 09:38:00AM +1200, Michael Schmitz wrote:
> That's one of the 'liberties' I alluded to. The reason I left these in is
> that I'm none too certain what device feature the DMA API uses to decide a
> device isn't cache-coherent.

The DMA API does not look at device features at all.  It needs to be
told so by the platform code.  Once an architecture implements the
hooks to support non-coherent DMA all devices are treated as
non-coherent by default unless overriden by the architecture either
globally (using the global dma_default_coherent variable) or per-device
(using the dev->dma_coherent field, usually set by arch_setup_dma_ops).

> If it's dev->coherent_dma_mask, the way I set
> up the device in the a3000 driver should leave the coherent mask unchanged.
> For the Zorro drivers, devices are set up to use the same storage to store
> normal and coherent masks - something we most likely want to change. I need
> to think about the ramifications of that.

No, the coherent mask is slightly misnamed amd not actually related.