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From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS
Date: Tue, 28 Jun 2022 23:25:42 -0700
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To: Michael Schmitz <schmitzmic@gmail.com>
Cc: Arnd Bergmann <arnd@kernel.org>, Geert Uytterhoeven <geert@linux-m68k.org>, scsi <linux-scsi@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, Jakub Kicinski <kuba@kernel.org>, Christoph Hellwig <hch@infradead.org>, Marek Szyprowski <m.szyprowski@samsung.com>, Robin Murphy <robin.murphy@arm.com>, Linux IOMMU <iommu@lists.linux-foundation.org>, Khalid Aziz <khalid@gonehiking.org>, "Maciej W . Rozycki" <macro@orcam.me.uk>, Matt Wang <wwentao@vmware.com>, Miquel van Smoorenburg <mikevs@xs4all.net>, Mark Salyzyn <salyzyn@android.com>, linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, Linux-Arch <linux-arch@vger.kernel.org>, alpha <linux-alpha@vger.kernel.org>, linux-m68k <linux-m68k@lists.linux>

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.