Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220617125750.728590-1-arnd@kernel.org> <20220617125750.728590-4-arnd@kernel.org>
 <6ba86afe-bf9f-1aca-7af1-d0d348d75ffc@gmail.com> <CAMuHMdVewn0OYA9oJfStk0-+vCKAUou+4Mvd5H2kmrSks1p5jg@mail.gmail.com>
 <b4e5a1c9-e375-63fb-ec7c-abb7384a6d59@gmail.com> <9289fd82-285c-035f-5355-4d70ce4f87b0@gmail.com>
In-Reply-To: <9289fd82-285c-035f-5355-4d70ce4f87b0@gmail.com>
From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Tue, 28 Jun 2022 09:03:53 +0200
Message-ID: <CAMuHMdXUihTPD9A9hs__Xr2ErfOqkZ5KgCHqm+9HvRf39uS5kA@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS
To:     Michael Schmitz <schmitzmic@gmail.com>
Cc:     Arnd Bergmann <arnd@kernel.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-m68k.org>,
        Parisc List <linux-parisc@vger.kernel.org>,
        Denis Efremov <efremov@linux.com>,
        Michael Ellerman <mpe@ellerman.id.au>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Hi Michael,

On Tue, Jun 28, 2022 at 5:26 AM Michael Schmitz <schmitzmic@gmail.com> wrote:
> Am 28.06.2022 um 09:12 schrieb Michael Schmitz:
> > On 27/06/22 20:26, Geert Uytterhoeven wrote:
> >> On Sat, Jun 18, 2022 at 3:06 AM Michael Schmitz <schmitzmic@gmail.com>
> >> wrote:
> >>> Am 18.06.2022 um 00:57 schrieb Arnd Bergmann:
> >>>> From: Arnd Bergmann <arnd@arndb.de>
> >>>>
> >>>> All architecture-independent users of virt_to_bus() and bus_to_virt()
> >>>> have been fixed to use the dma mapping interfaces or have been
> >>>> removed now.  This means the definitions on most architectures, and the
> >>>> CONFIG_VIRT_TO_BUS symbol are now obsolete and can be removed.
> >>>>
> >>>> The only exceptions to this are a few network and scsi drivers for m68k
> >>>> Amiga and VME machines and ppc32 Macintosh. These drivers work
> >>>> correctly
> >>>> with the old interfaces and are probably not worth changing.
> >>> The Amiga SCSI drivers are all old WD33C93 ones, and replacing
> >>> virt_to_bus by virt_to_phys in the dma_setup() function there would
> >>> cause no functional change at all.
> >> FTR, the sgiwd93 driver use dma_map_single().
> >
> > Thanks! From what I see, it doesn't have to deal with bounce buffers
> > though?
>
> 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).

Thanks, looks reasonable.

> The driver allocates bounce buffers using kmalloc if it hits an
> unaligned data buffer - can such buffers still even happen these days?

No idea.

> 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?
>
> Thinking ahead to the other two Amiga drivers - I wonder whether
> allocating a static bounce buffer or a DMA pool at driver init is likely
> to succeed if the kernel runs from the low 16 MB RAM chunk? It certainly
> won't succeed if the kernel runs from a higher memory address, so the
> present bounce buffer logic around amiga_chip_alloc() might still need
> to be used here.
>
> Leaves the question whether converting the gvp11 and a2091 drivers is
> actually worth it, if bounce buffers still have to be handled explicitly.

A2091 should be straight-forward, as A3000 is basically A2091 on the
motherboard (comparing the two drivers, looks like someone's been
sprinkling mb()s over the A3000 driver).

I don't have any of these SCSI host adapters (not counting the A590
(~A2091) expansion of the old A500, which is not Linux-capable, and
 hasn't been powered on for 20 years).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds