linux-kernel - RE: [PATCH v3 03/13] pm: at91: Workaround DDRSDRC self-refresh bug with LPDDR1 memories.

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Date:	Tue, 27 Jan 2015 12:14:11 +0000
From:	Peter Rosin <peda@...ntia.se>
To:	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	Wenyou Yang <wenyou.yang@...el.com>,
	"nicolas.ferre@...el.com" <nicolas.ferre@...el.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"alexandre.belloni@...e-electrons.com" 
	<alexandre.belloni@...e-electrons.com>,
	"sylvain.rochet@...secur.com" <sylvain.rochet@...secur.com>,
	"linux@...im.org.za" <linux@...im.org.za>
Subject: RE: [PATCH v3 03/13] pm: at91: Workaround DDRSDRC self-refresh bug
 with LPDDR1 memories.

Sergei Shtylyov wrote:
> Hello.

Hi!

> On 1/27/2015 8:53 AM, Wenyou Yang wrote:
> 
> > From: Peter Rosin <peda@...ntia.se>
> 
> > The DDRSDR controller fails miserably to put LPDDR1 memories in
> > self-refresh. Force the controller to think it has DDR2 memories
> > during the self-refresh period, as the DDR2 self-refresh spec is
> > equivalent to LPDDR1, and is correctly implemented in the controller.
> 
> > Assume that the second controller has the same fault, but that is
> > untested.
> 
> > Signed-off-by: Peter Rosin <peda@...ntia.se>
> > Acked-by: Nicolas Ferre <nicolas.ferre@...el.com>
> > ---
> >   arch/arm/mach-at91/pm_slowclock.S  |   43
> +++++++++++++++++++++++++++++++-----
> >   include/soc/at91/at91sam9_ddrsdr.h |    2 +-
> >   2 files changed, 39 insertions(+), 6 deletions(-)
> 
> > diff --git a/arch/arm/mach-at91/pm_slowclock.S
> > b/arch/arm/mach-at91/pm_slowclock.S
> > index e2bfaf5..1155217 100644
> > --- a/arch/arm/mach-at91/pm_slowclock.S
> > +++ b/arch/arm/mach-at91/pm_slowclock.S
> [...]
> > @@ -108,14 +118,26 @@ ddr_sr_enable:
> >
> >   	/* figure out if we use the second ram controller */
> >   	cmp	ramc1, #0
> > -	ldrne	tmp2, [ramc1, #AT91_DDRSDRC_LPR]
> > -	strne	tmp2, .saved_sam9_lpr1
> > -	bicne	tmp2, #AT91_DDRSDRC_LPCB
> > -	orrne	tmp2, #AT91_DDRSDRC_LPCB_SELF_REFRESH
> > +	beq	ddr_no_2nd_ctrl
> > +
> > +	ldr	tmp2, [ramc1, #AT91_DDRSDRC_MDR]
> > +	str	tmp2, .saved_sam9_mdr1
> > +	bic	tmp2, tmp2, #~AT91_DDRSDRC_MD
> > +	cmp	tmp2, #AT91_DDRSDRC_MD_LOW_POWER_DDR
> > +	ldreq	tmp2, [ramc1, #AT91_DDRSDRC_MDR]
> > +	biceq	tmp2, tmp2, #AT91_DDRSDRC_MD
> 
>     Didn't you forget ~? Either that, or ~ above is not needed, I think.

The code is correct, the first bic with ~ clears bits not in the relevant
field in order to compare if LPDDR mode is active. The second bic(eq)
w/o ~ clears the field, to make way for the bits in the below orreq
when actually changing the register content into DDR2 mode.

> > +	orreq	tmp2, tmp2, #AT91_DDRSDRC_MD_DDR2
> > +	streq	tmp2, [ramc1, #AT91_DDRSDRC_MDR]
> > +
> > +	ldr	tmp2, [ramc1, #AT91_DDRSDRC_LPR]
> > +	str	tmp2, .saved_sam9_lpr1
> > +	bic	tmp2, #AT91_DDRSDRC_LPCB
> 
>     Didn't you forget ~? And isn't it 3-operand instruction (as seen in the above
> code)?

The logic for the LPR register is from the old code, the "only" thing
I did to it was changing the instruction sequence to not have the
???ne form, i.e. ldrne, strne, bicne, orrne became ldr, str, bic, orr
with a jump around it instead. So, the original code also had a two
argument bic(ne), which indeed is strange, and I don't know why
there is no warning from the assembler. Since there is no warning,
my guess is that the assembler somehow mends it? Or does the
patch actually break the second controller? It would be a surprise
it the assembler handles 2-operand bicne differently from a
2-operand bic, no?

> > +	orr	tmp2, #AT91_DDRSDRC_LPCB_SELF_REFRESH
> 
>     Only 2 operands?

Same argument as above. I didn't touch it (sort of...)

Should I update the patch and fix this collateral 2-operand problem as
well? To me, it feels like a separate patch, no?

> [...]
> 
> WBR, Sergei

Cheers,
Peter
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