From: David Laight <David.Laight@ACULAB.COM>
To: 'Christophe Leroy' <christophe.leroy@csgroup.eu>,
'Michael Schmitz' <schmitzmic@gmail.com>,
Arnd Bergmann <arnd@kernel.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>,
Linux-Arch <linux-arch@vger.kernel.org>,
scsi <linux-scsi@vger.kernel.org>,
"Christoph Hellwig" <hch@infradead.org>,
Geert Uytterhoeven <geert@linux-m68k.org>,
"Jakub Kicinski" <kuba@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
Denis Efremov <efremov@linux.com>,
linux-m68k <linux-m68k@lists.linux-m68k.org>,
"John Paul Adrian Glaubitz" <glaubitz@physik.fu-berlin.de>,
Khalid Aziz <khalid@gonehiking.org>,
Miquel van Smoorenburg <mikevs@xs4all.net>,
"Parisc List" <linux-parisc@vger.kernel.org>,
Robin Murphy <robin.murphy@arm.com>,
Matt Wang <wwentao@vmware.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Mark Salyzyn <salyzyn@android.com>,
"Linux IOMMU" <iommu@lists.linux-foundation.org>,
alpha <linux-alpha@vger.kernel.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
"Maciej W . Rozycki" <macro@orcam.me.uk>
Subject: RE: [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS
Date: Thu, 30 Jun 2022 10:32:23 +0000 [thread overview]
Message-ID: <8bedba3ddffc435ea44a5e2893583acb@AcuMS.aculab.com> (raw)
In-Reply-To: <a36a85a3-3fd3-10ac-cac3-09a90eaf1936@csgroup.eu>
From: Christophe Leroy
> Sent: 30 June 2022 10:40
>
> Le 30/06/2022 à 10:04, David Laight a écrit :
> > From: Michael Schmitz
> >> Sent: 29 June 2022 00:09
> >>
> >> Hi Arnd,
> >>
> >> On 29/06/22 09:50, Arnd Bergmann wrote:
> >>> On Tue, Jun 28, 2022 at 11:03 PM Michael Schmitz <schmitzmic@gmail.com> wrote:
> >>>> On 28/06/22 19:03, Geert Uytterhoeven wrote:
> >>>>>> 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.
> >>> kmalloc() guarantees alignment to the next power-of-two size or
> >>> KMALLOC_MIN_ALIGN, whichever is bigger. On m68k this means it
> >>> is cacheline aligned.
> >>
> >> And all SCSI buffers are allocated using kmalloc? No way at all for user
> >> space to pass unaligned data?
> >
> > I didn't think kmalloc() gave any such guarantee about alignment.
>
> I does since commit 59bb47985c1d ("mm, sl[aou]b: guarantee natural
> alignment for kmalloc(power-of-two)")
Looks like it is done for 'power-of-two' less than PAGE_SIZE.
This may not help scsi tape writes which could easily be (say) 47 bytes.
I think that only guarantees 2 byte alignment on m68k.
(Although increasing the min-alignment on m68k to 4 (or even 8)
will probably make no measurable difference.)
What happens above PAGE_SIZE?
Any structure with a trailing [] field could easily request
'64k + a_bit' bytes.
You don't really want to extend this to 128k - but I suspect
that is what happens.
David
>
> Christophe
>
> > There are cache-line alignment requirements on systems with non-coherent
> > dma, but otherwise the alignment can be much smaller.
> >
> > One of the allocators adds a header to each item, IIRC that can
> > lead to 'unexpected' alignments - especially on m68k.
> >
> > dma_alloc_coherent() does align to next 'power of 2'.
> > And sometimes you need (eg) 16k allocates that are 16k aligned.
> >
> > David
> >
> > -
> > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> > Registration No: 1397386 (Wales)
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2022-06-30 10:33 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-17 12:57 [PATCH v2 0/3] phase out CONFIG_VIRT_TO_BUS Arnd Bergmann
2022-06-17 12:57 ` [PATCH v2 1/3] scsi: dpt_i2o: drop stale VIRT_TO_BUS dependency Arnd Bergmann
2022-06-21 8:43 ` Hannes Reinecke
2022-06-17 12:57 ` [PATCH v2 2/3] scsi: BusLogic remove bus_to_virt Arnd Bergmann
2022-06-17 14:02 ` Robin Murphy
2022-06-21 8:45 ` Hannes Reinecke
2022-06-21 21:56 ` Khalid Aziz
2022-06-23 14:47 ` Arnd Bergmann
2022-06-24 15:38 ` Khalid Aziz
2022-06-24 15:43 ` Arnd Bergmann
2022-06-17 12:57 ` [PATCH v2 3/3] arch/*/: remove CONFIG_VIRT_TO_BUS Arnd Bergmann
2022-06-18 1:06 ` Michael Schmitz
2022-06-24 9:10 ` Arnd Bergmann
2022-06-26 5:21 ` Michael Schmitz
2022-06-26 8:36 ` Arnd Bergmann
2022-06-27 8:09 ` Michael Schmitz
2022-06-27 8:26 ` Geert Uytterhoeven
2022-06-27 21:12 ` Michael Schmitz
2022-06-28 3:25 ` Michael Schmitz
2022-06-28 7:03 ` Geert Uytterhoeven
2022-06-28 21:03 ` Michael Schmitz
2022-06-28 21:50 ` Arnd Bergmann
2022-06-28 23:09 ` Michael Schmitz
2022-06-28 23:50 ` Bart Van Assche
2022-06-29 0:01 ` Michael Schmitz
2022-06-29 0:14 ` Michael Schmitz
2022-06-29 6:21 ` Christoph Hellwig
2022-06-30 19:21 ` Michael Schmitz
2022-06-30 8:04 ` David Laight
2022-06-30 9:40 ` Christophe Leroy
2022-06-30 10:32 ` David Laight [this message]
2022-06-28 7:08 ` Arnd Bergmann
2022-06-28 21:38 ` Michael Schmitz
2022-06-28 21:55 ` Arnd Bergmann
2022-06-28 23:43 ` Michael Schmitz
2022-06-29 6:25 ` Christoph Hellwig
2022-06-30 19:26 ` Michael Schmitz
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