Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS

From: Michael Schmitz <schmitzmic@gmail.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
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>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Subject: Re: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS
Date: Wed, 29 Jun 2022 09:03:37 +1200	[thread overview]
Message-ID: <c30bc9b6-6ccd-8856-dc6b-4e16450dad6f@gmail.com> (raw)
In-Reply-To: <CAMuHMdXUihTPD9A9hs__Xr2ErfOqkZ5KgCHqm+9HvRf39uS5kA@mail.gmail.com>

Hi Geert,

On 28/06/22 19:03, Geert Uytterhoeven wrote:
>
>> 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.
Thanks, I'll take care of Arnd's comments and post a corrected version 
later.
>> The driver allocates bounce buffers using kmalloc if it hits an
>> unaligned data buffer - can such buffers still even happen these days?
> No idea.
Hmmm - I think I'll stick a WARN_ONCE() in there so we know whether this 
code path is still being used.
>
>> 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).

Yep, and at least the ones in the dma_setup() function are there for no 
reason (the compiler won't reorder stores around the cache flush calls, 
I hope?).

Just leaves the 24 bit DMA mask there (and likely need for bounce buffers).

> 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).

I wonder whether kullervo has survived - that one was an A3000. Should 
have gone to Adrian a few years ago...

Cheers,

     Michael

>
> 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