[U-Boot] [PATCH 1/4] ARM: pxa: prevent PXA270 occasional reboot freezes

[Marek Vasut <marex@denx.de>]

On Saturday, December 14, 2013 at 04:31:31 PM, Sergei Ianovich wrote:
> On Sat, 2013-12-14 at 13:29 +0100, Marek Vasut wrote:
> > On Saturday, December 14, 2013 at 06:41:57 AM, Sergei Ianovich wrote:
> > > Erratum 71 of PXA270M Processor Family Specification Update
> > > (April 19, 2010) explains that watchdog reset time is just
> > > 8us insead of 10ms in EMTS.
> > > 
> > > If SDRAM is not reset, it causes memory bus congestion and
> > > the device hangs.
> > > 
> > > We put SDRAM in selfresh mode before watchdog reset, removing
> > > potential freezes.
> > > 
> > > Signed-off-by: Sergei Ianovich <ynvich@gmail.com>
> > > ---
> > > 
> > >  arch/arm/cpu/pxa/pxa2xx.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm/cpu/pxa/pxa2xx.c b/arch/arm/cpu/pxa/pxa2xx.c
> > > index c9a7d45..93ca2f0 100644
> > > --- a/arch/arm/cpu/pxa/pxa2xx.c
> > > +++ b/arch/arm/cpu/pxa/pxa2xx.c
> > > @@ -281,5 +281,5 @@ void reset_cpu(ulong ignored)
> > > 
> > >       writel(tmp, OSMR3);
> > >       
> > >       for (;;)
> > > 
> > > -             ;
> > > +             writel(MDREFR_SLFRSH, MDREFR);
> > 
> > Do you need to write this register in an endless loop ?
> 
> I didn't think this way. We need to have at least 3, but up to 5 cycles
> to put SDRAM in SLFRFRSH. It depends on the current state of SDRAM.
> There is no way to know.

OK, I seem to remember the uglinesses of the PXA DRAM controller, indeed :(

> It can probably work if we write just once. But if we have another
> thread doing something with SDRAM in between, we will still hang.

U-Boot is single-threaded ;-)

> I am not sure how likely is the situation, though.

It cannot happen, really ;-)

BUT (!) I understand your intention. If writing the MDREFR multiple times won't 
be a problem, I am _not_ opposed to this patch. So please only make sure that's 
not a problem and if it's not, I won't block this patch.

Thank you very much for your efforts on the PXA front !

Best regards,
Marek Vasut