Re: [PATCH] MIPS: reloc: make flush_cache_all() usable

[Sascha Hauer <s.hauer@pengutronix.de>]

On Fri, Jun 28, 2019 at 11:35:45AM +0300, Antony Pavlov wrote:
> On Wed, 26 Jun 2019 08:53:54 +0200
> Sascha Hauer <s.hauer@pengutronix.de> wrote:
> 
> > Hi Antony,
> > 
> > On Fri, Jun 21, 2019 at 08:15:07AM +0300, Antony Pavlov wrote:
> > > flush_cache_all() uses 'struct cpuinfo_mips current_cpu_data' data fields.
> > > These data fields are initialized in r4k_cache_init().
> > > 
> > > However in the current implementation the r4k_cache_init() function
> > > is called __AFTER__ relocate_code().
> > > 
> > > Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
> > > ---
> > >  arch/mips/lib/reloc.c | 12 ++++++++++++
> > >  1 file changed, 12 insertions(+)
> > > 
> > > diff --git a/arch/mips/lib/reloc.c b/arch/mips/lib/reloc.c
> > > index 14ba6167dd..6f1cd6d82f 100644
> > > --- a/arch/mips/lib/reloc.c
> > > +++ b/arch/mips/lib/reloc.c
> > > @@ -37,6 +37,7 @@
> > >  #include <asm/mipsregs.h>
> > >  #include <asm/relocs.h>
> > >  #include <asm/sections.h>
> > > +#include <asm/cpu-features.h>
> > >  #include <linux/sizes.h>
> > >  #include <asm-generic/memory_layout.h>
> > >  
> > > @@ -157,6 +158,17 @@ void relocate_code(void *fdt, u32 fdt_size, u32 ram_size)
> > >  		apply_reloc(type, (void *)addr, off);
> > >  	}
> > >  
> > > +	/* clear the BSS first */
> > > +	memset(__bss_start, 0x00, bss_len);
> > > +
> > > +	cpu_probe();
> > > +
> > > +	if (cpu_has_4k_cache) {
> > > +		extern void r4k_cache_init(void);
> > > +
> > > +		r4k_cache_init();
> > > +	}
> > > +
> > >  	/* Ensure the icache is coherent */
> > >  	flush_cache_all();
> > 
> > Can we initialize one field in current_cpu_data to something nonzero so
> > that it goes out of the bss?
> 
> Or clear only current_cpu_data and keep it in bss?

That would at least make it clear why this is done.

That's important because otherwise some day somebody sends a patch
"mips: remove duplicate clearing of bss" and I might apply it because
it looks correct ;)

There might be one problem with putting current_cpu_data into bss. I
don't know much about MIPS booting, but in case the initial image is
located in some potentially small SRAM then putting current_cpu_data
into bss means that the SRAM has to be big enough for initial image and
bss.

> 
> > Another way would be to pass cpu_probe() a pointer to a struct
> > cpuinfo_mips allocated on the stack and then pass it to
> > flush_cache_all().
> > 
> > Clearing the bss twice doesn't look very nice.
> 
> It's two diffrent bss segments: one before relocation and one after relocation.

Yes, I know, otherwise you could just drop the second bss clearing.

Sascha

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