linux-kernel - Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO

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Date:   Tue, 28 Jun 2022 23:25:42 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Michael Schmitz <schmitzmic@...il.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        scsi <linux-scsi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Jakub Kicinski <kuba@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        Khalid Aziz <khalid@...ehiking.org>,
        "Maciej W . Rozycki" <macro@...am.me.uk>,
        Matt Wang <wwentao@...are.com>,
        Miquel van Smoorenburg <mikevs@...all.net>,
        Mark Salyzyn <salyzyn@...roid.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        alpha <linux-alpha@...r.kernel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        Denis Efremov <efremov@...ux.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS

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.