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

Christophe Leroy
2022-06-30T09:40:31Z

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)")

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
>
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