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

[David Laight]

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@xxxxxxxxx> 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.
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

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