linux-kernel - Re: [PATCH 2/2] power/reset: at91-poweroff: timely shitdown LPDDR memories

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Date:   Thu, 13 Oct 2016 15:47:10 +0200
From:   Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:     Jean-Jacques Hiblot <jjhiblot@...phandler.com>
Cc:     Sebastian Reichel <sre@...nel.org>,
        Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
        Nicolas Ferre <nicolas.ferre@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, linux-pm@...r.kernel.org
Subject: Re: [PATCH 2/2] power/reset: at91-poweroff: timely shitdown LPDDR
 memories

On 13/10/2016 at 14:27:15 +0200, Jean-Jacques Hiblot wrote :
> 2016-10-13 13:03 GMT+02:00 Alexandre Belloni
> <alexandre.belloni@...e-electrons.com>:
> > On 12/10/2016 at 14:48:27 +0200, Jean-Jacques Hiblot wrote :
> >> > +static void at91_lpddr_poweroff(void)
> >> > +{
> >> > +       asm volatile(
> >> > +               /* Align to cache lines */
> >> > +               ".balign 32\n\t"
> >> > +
> >> > +               "       ldr     r6, [%2, #" __stringify(AT91_SHDW_CR) "]\n\t"
> >> At first sight, it looks useless. I assume it's used to preload the
> >> TLB before the LPDDR is turned off.
> >> A comment to explain why this line is useful would prevent its removal.
> >
> > Yes, this is the case. I can add a comment.
> >
> > Anyway, I would prefer the whole thing to run from SRAM, as a PIE
> > instead of relying on the cache.
> 
> Instead of copying into the SRAM, you can make the cache reliable by
> preloading it, much like the TLB.
> LDI is probably not available for most of atmel's SOC, so the only way
> I can think of, is to execute code from the targeted area. here is an
> example:
> +               /*
> +                * Jump to the end of the sequence to preload instruction cache
> +                * It only works because the sequence is short enough not to
> +                * sit accross more than 2 cache lines
> +                */
> +               "       b end_of_sequence\n\t"
> +               "start_of_sequence:\n\t"
> +
>                 /* Power down SDRAM0 */
>                 "       str     %1, [%0, #"
> __stringify(AT91_DDRSDRC_LPR) "]\n\t"
>                 /* Shutdown CPU */
>                 "       str     %3, [%2, #" __stringify(AT91_SHDW_CR) "]\n\t"
> 
>                 "       b       .\n\t"
> +
> +               /*
> +                * we're now 100% sure that the code to shutdown the LPDDR and
> +                * the CPU is in cache, go back to do the actual job
> +                */
> +               "end_of_sequence:\n\t"
> +               "       b start_of_sequence\n\t"
>                 :
> 

I don't think this is necessary. By aligning the instructions properly,
we are already sure the whole code is loaded into the cache.

My plan is to get rid of the assembly and use PIE so it is written in C
and we can properly separate the RAM stuff in the ddrc driver.

The mpddrc driver could load its shutdown function in SRAM. The reset
controller driver would load the reset function in SRAM and the shutdown
controller would load the poweroff function in SRAM. It would e quite
cleaner than what we have here.

-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com