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

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Date:   Tue, 28 Jun 2022 09:08:14 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Michael Schmitz <schmitzmic@...il.com>
Cc:     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>
Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS

On Tue, Jun 28, 2022 at 5:25 AM Michael Schmitz <schmitzmic@...il.com> wrote:
> Am 28.06.2022 um 09:12 schrieb Michael Schmitz:
>
> Leaving the bounce buffer handling in place, and taking a few other
> liberties - this is what converting the easiest case (a3000 SCSI) might
> look like. Any obvious mistakes? The mvme147 driver would be very
> similar to handle (after conversion to a platform device).
>
> The driver allocates bounce buffers using kmalloc if it hits an
> unaligned data buffer - can such buffers still even happen these days?
> If I understand dma_map_single() correctly, the resulting dma handle
> would be equally misaligned?
>
> To allocate a bounce buffer, would it be OK to use dma_alloc_coherent()
> even though AFAIU memory used for DMA buffers generally isn't consistent
> on m68k?

I think it makes sense to skip the bounce buffering as you do here:
the only standardized way we have for integrating that part is to
use the swiotlb infrastructure, but as you mentioned earlier that
part is probably too resource-heavy here for Amiga.

I see two other problems with your patch though:

a) you still duplicate the cache handling: the cache_clear()/cache_push()
is supposed to already be done by dma_map_single() when the device
is not cache-coherent.

b) The bounce buffer is never mapped here, instead you have the
virt_to_phys() here, which is not the same. I think you need to map
the pointer that actually gets passed down to the device after deciding
to use a bouce buffer or not.

     Arnd