Subject: Re: [PATCH] powerpc: ensure that swiotlb buffer is allocated from low memory

From: Mike Rapoport <[email-masked]>
Date: Mon, 9 Dec 2019 09:54:15 +0200

On Mon, Dec 09, 2019 at 04:43:17PM +1100, Michael Ellerman wrote:
> Mike Rapoport <[email-masked]> writes:
> > From: Mike Rapoport <[email-masked]>
> >
> > Some powerpc platforms (e.g. 85xx) limit DMA-able memory way below 4G. If a
> > system has more physical memory than this limit, the swiotlb buffer is not
> > addressable because it is allocated from memblock using top-down mode.
> >
> > Force memblock to bottom-up mode before calling swiotlb_init() to ensure
> > that the swiotlb buffer is DMA-able.
> >
> > Link: https://lkml.kernel.org/r/[email-masked]
> 
> This wasn't bisected, but I thought it was a regression. Do we know what
> commit caused it?
> 
> Was it 25078dc1f74b ("powerpc: use mm zones more sensibly") ?

swiotlb buffer is initialized before zones are actually used, so probably
not :)
 
> Or was that a red herring?
> 
> cheers
> 
> > Reported-by: Christian Zigotzky <[email-masked]>
> > Signed-off-by: Mike Rapoport <[email-masked]>
> > Cc: Benjamin Herrenschmidt <[email-masked]>
> > Cc: Christoph Hellwig <[email-masked]>
> > Cc: Darren Stevens <[email-masked]>
> > Cc: mad skateman <[email-masked]>
> > Cc: Michael Ellerman <[email-masked]>
> > Cc: Nicolas Saenz Julienne <[email-masked]>
> > Cc: Paul Mackerras <[email-masked]>
> > Cc: Robin Murphy <[email-masked]>
> > Cc: Rob Herring <[email-masked]>
> > ---
> >  arch/powerpc/mm/mem.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
> > index be941d382c8d..14c2c53e3f9e 100644
> > --- a/arch/powerpc/mm/mem.c
> > +++ b/arch/powerpc/mm/mem.c
> > @@ -260,6 +260,14 @@ void __init mem_init(void)
> >  	BUILD_BUG_ON(MMU_PAGE_COUNT > 16);
> >  
> >  #ifdef CONFIG_SWIOTLB
> > +	/*
> > +	 * Some platforms (e.g. 85xx) limit DMA-able memory way below
> > +	 * 4G. We force memblock to bottom-up mode to ensure that the
> > +	 * memory allocated in swiotlb_init() is DMA-able.
> > +	 * As it's the last memblock allocation, no need to reset it
> > +	 * back to to-down.
> > +	 */
> > +	memblock_set_bottom_up(true);
> >  	swiotlb_init(0);
> >  #endif
> >  
> > -- 
> > 2.24.0

-- 
Sincerely yours,
Mike.