Re: [PATCH v5 15/15] arm64: Enable ARCH_WANT_KMALLOC_DMA_BOUNCE for arm64

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, May 25, 2023 at 05:12:37PM +0100, Robin Murphy wrote:
> On 24/05/2023 6:19 pm, Catalin Marinas wrote:
> > With the DMA bouncing of unaligned kmalloc() buffers now in place,
> > enable it for arm64 to allow the kmalloc-{8,16,32,48,96} caches. In
> > addition, always create the swiotlb buffer even when the end of RAM is
> > within the 32-bit physical address range (the swiotlb buffer can still
> > be disabled on the kernel command line).
> > 
> > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> > Cc: Will Deacon <will@kernel.org>
> > ---
> >   arch/arm64/Kconfig   | 1 +
> >   arch/arm64/mm/init.c | 7 ++++++-
> >   2 files changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index b1201d25a8a4..af42871431c0 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -120,6 +120,7 @@ config ARM64
> >   	select CRC32
> >   	select DCACHE_WORD_ACCESS
> >   	select DYNAMIC_FTRACE if FUNCTION_TRACER
> > +	select DMA_BOUNCE_UNALIGNED_KMALLOC
> 
> We may want to give the embedded folks an easier way of turning this off,
> since IIRC one of the reasons for the existing automatic behaviour was
> people not wanting to have to depend on the command line. Things with 256MB
> or so of RAM seem unlikely to get enough memory efficiency back from the
> smaller kmem caches to pay off the SWIOTLB allocation :)

I thought about this initially and that's why I had two options
(ARCH_WANT_* and this one). But we already select SWIOTLB on arm64, so
for the embedded folk the only option is swiotlb=noforce on the cmdline
which, in turn, limits the kmalloc caches to kmalloc-64 (or whatever the
cache line size is) irrespective of this new select.

-- 
Catalin