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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