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

[David Laight <David.Laight-ZS65k/vG3HxXrIkS9f7CXA@public.gmane.org>]

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)
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