Re: [PATCH] irqchip/gic-v3-its: Add early memory allocation errata

[Marc Zyngier]

Hi Matthias,

On 04/10/18 23:11, Matthias Brugger wrote:
> Friendly reminder, if anyone has any comment on the patch :)
>
> On 9/12/18 11:52 AM, matthias.bgg@xxxxxxxxxx wrote:
> > From: Matthias Brugger <mbrugger@xxxxxxxx>
> >
> > Some hardware does not implement two-level page tables so that
> > the amount of contigious memory needed by the baser is bigger
> > then the zone order. This is a known problem on Cavium Thunderx
> > with 4K page size.
> >
> > We fix this by adding an errata which allocates the memory early
> > in the boot cycle, using the memblock allocator.
> >
> > Signed-off-by: Matthias Brugger <mbrugger@xxxxxxxx>
> > ---
> >   arch/arm64/Kconfig               | 12 ++++++++
> >   arch/arm64/include/asm/cpucaps.h |  3 +-
> >   arch/arm64/kernel/cpu_errata.c   | 33 +++++++++++++++++++++
> >   drivers/irqchip/irq-gic-v3-its.c | 50 ++++++++++++++++++++------------
> >   4 files changed, 79 insertions(+), 19 deletions(-)

My only comment would be to state how much I dislike both the HW and the 
patch... ;-) The idea that we have some erratum that depends on the page 
size doesn't feel good at all.

> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 1b1a0e95c751..dfd9fe08f0b2 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -597,6 +597,18 @@ config QCOM_FALKOR_ERRATUM_E1041
> >   
> >   	  If unsure, say Y.
> >   
> > +config CAVIUM_ALLOC_ITS_TABLE_EARLY
> > +	bool "Cavium Thunderx: Allocate the its table early"
> > +	default y
> > +	depends on ARM64_4K_PAGES && FORCE_MAX_ZONEORDER < 13

Here's a though: Why don't we ensure that FORCE_MAX_ZONEORDER is such as 
we could always allocate the same amount of memory, no matter what the 
page size is? That, or bump FORCE_MAX_ZONEORDER to 13 if the kernel 
includes support for TX1.

Any of this of course requires buy-in from the arm64 maintainers, as 
this is quite a departure from the way things work so far.

Thanks,
	
	M.
--
Jazz is not dead. It just smells funny...