linux-kernel - Re: [PATCH 2/2] powerpc/83xx: map IMMR with a BAT.

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Message-ID: <fb07c36767c1b4c6bacfd4bb0fddf789d248573e.camel@buserror.net>
Date:   Sun, 15 Sep 2019 12:09:40 -0500
From:   Scott Wood <oss@...error.net>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        galak@...nel.crashing.org
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/2] powerpc/83xx: map IMMR with a BAT.

On Sat, 2019-09-14 at 18:51 +0200, Christophe Leroy wrote:
> 
> Le 14/09/2019 à 16:34, Scott Wood a écrit :
> > On Fri, 2019-08-23 at 12:50 +0000, Christophe Leroy wrote:
> > > On mpc83xx with a QE, IMMR is 2Mbytes.
> > > On mpc83xx without a QE, IMMR is 1Mbytes.
> > > Each driver will map a part of it to access the registers it needs.
> > > Some driver will map the same part of IMMR as other drivers.
> > > 
> > > In order to reduce TLB misses, map the full IMMR with a BAT.
> > > 
> > > Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> > > ---
> > >   arch/powerpc/platforms/83xx/misc.c | 10 ++++++++++
> > >   1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/arch/powerpc/platforms/83xx/misc.c
> > > b/arch/powerpc/platforms/83xx/misc.c
> > > index f46d7bf3b140..1e395b01c535 100644
> > > --- a/arch/powerpc/platforms/83xx/misc.c
> > > +++ b/arch/powerpc/platforms/83xx/misc.c
> > > @@ -18,6 +18,8 @@
> > >   #include <sysdev/fsl_soc.h>
> > >   #include <sysdev/fsl_pci.h>
> > >   
> > > +#include <mm/mmu_decl.h>
> > > +
> > >   #include "mpc83xx.h"
> > >   
> > >   static __be32 __iomem *restart_reg_base;
> > > @@ -145,6 +147,14 @@ void __init mpc83xx_setup_arch(void)
> > >   	if (ppc_md.progress)
> > >   		ppc_md.progress("mpc83xx_setup_arch()", 0);
> > >   
> > > +	if (!__map_without_bats) {
> > > +		int immrsize = IS_ENABLED(CONFIG_QUICC_ENGINE) ? SZ_2M :
> > > SZ_1M;
> > 
> > Any reason not to unconditionally make it 2M?  After all, the kernel being
> > built with CONFIG_QUICC_ENGINE doesn't mean that the hardware you're
> > running
> > on has it...
> > 
> 
> Euh .. ok. I didn't see it that way, but you are right.
> 
> Do you think it is not a problem to map 2M even when the quicc engine is 
> not there ? Or should it check device tree instead ?

It should be OK, since it's a guarded mapping.  Unless the IMMR base is not 2M
aligned...

-Scott