[PATCH] arm64: swiotlb: Reduce the default size if no ZONE_DMA bouncing needed

[Catalin Marinas]

On Fri, Oct 06, 2023 at 05:25:03PM +0100, Robin Murphy wrote:
> On 2023-10-06 16:32, Catalin Marinas wrote:
> > With CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC enabled, the arm64 kernel still
> > allocates the default SWIOTLB buffer (64MB) even if ZONE_DMA is disabled
> > or all the RAM fits into this zone. However, this potentially wastes a
> > non-negligible amount of memory on platforms with little RAM.
> > 
> > Reduce the SWIOTLB size to 1MB per 1GB of RAM if only needed for
> > kmalloc() buffer bouncing.
> > 
> > Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> > Suggested-by: Ross Burton <ross.burton at arm.com>
> > Cc: Ross Burton <ross.burton at arm.com>
> > Cc: Robin Murphy <robin.murphy at arm.com>
> > Cc: Will Deacon <will at kernel.org>
> > ---
> >   arch/arm64/mm/init.c | 10 +++++++++-
> >   1 file changed, 9 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > index 8a0f8604348b..54ee1a4868c2 100644
> > --- a/arch/arm64/mm/init.c
> > +++ b/arch/arm64/mm/init.c
> > @@ -493,8 +493,16 @@ void __init mem_init(void)
> >   {
> >   	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
> > -	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC))
> > +	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb) {
> > +		/*
> > +		 * If no bouncing needed for ZONE_DMA, reduce the swiotlb
> > +		 * buffer for kmalloc() bouncing to 1MB per 1GB of RAM.
> > +		 */
> > +		unsigned long size =
> > +			ALIGN(memblock_phys_mem_size(), SZ_1G) >> 10;
> 
> Hmm, I wondered if DIV_ROUND_UP(memblock_phys_mem_size(), 1024) might be any
> easier, but by the time I've typed it out it's still just as long :)

It's not the same result as it's rounded up to the nearest KB rather
than MB. But this may work as well. I just made up this number really.

-- 
Catalin