linux-kernel - Re: [PATCH] ARM: EXYNOS: mcpm: Don't rely on firmware's secondary_cpu

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Date:	Sat, 7 Jun 2014 19:12:21 +0100
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Doug Anderson <dianders@...omium.org>
Cc:	Kukjin Kim <kgene.kim@...sung.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Abhilash Kesavan <a.kesavan@...sung.com>,
	Andrew Bresticker <abrestic@...omium.org>,
	Inderpal Singh <inderpal.s@...sung.com>,
	Thomas Abraham <thomas.ab@...sung.com>,
	"olof@...om.net" <olof@...om.net>,
	Tushar Behera <trblinux@...il.com>,
	Kevin Hilman <khilman@...aro.org>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: EXYNOS: mcpm: Don't rely on firmware's
 secondary_cpu_start

On Fri, Jun 06, 2014 at 10:43:05PM +0100, Doug Anderson wrote:
> On exynos mcpm systems the firmware is hardcoded to jump to an address
> in SRAM (0x02073000) when secondary CPUs come up.  By default the
> firmware puts a bunch of code at that location.  That code expects the
> kernel to fill in a few slots with addresses that it uses to jump back
> to the kernel's entry point for secondary CPUs.
> 
> Originally (on prerelease hardware) this firmware code contained a
> bunch of workarounds to deal with boot ROM bugs.  However on all
> shipped hardware we simply use this code to redirect to a kernel
> function for bringing up the CPUs.
> 
> Let's stop relying on the code provided by the bootloader and just
> plumb in our own (simple) code jump to the kernel.  This has the nice
> benefit of fixing problems due to the fact that older bootloaders
> (like the one shipped on the Samsung Chromebook 2) might have put
> slightly different code into this location.
> 
> Once suspend/resume is implemented for systems using exynos-mcpm we'll
> need to make sure we reinstall our fixed up code after resume.  ...but
> that's not anything new since IRAM (and thus the address of the
> mcpm_entry_point) is lost across suspend/resume anyway.

Can I ask you please what the firmware does for the boot (primary) cpu
on cold-boot and warm-boot (resume from suspend) ?

Does it jump to a specific (hardcoded) location ?

Is the primary CPU (MPIDR) hardcoded in firmware so that on both
cold and warm-boot firmware sees a specific MPIDR as "special" ?

I am asking to check if on this platform CPUidle (where the notion of
primary CPU disappears) has a chance to run properly.

Probably CPUidle won't attain idle states where IRAM content is lost, but I
am still worried about the primary vs secondaries firmware boot behaviour.

What happens on reboot from suspend to RAM (or to put it differently,
what does secure firmware do on reboot from suspend to RAM - in
particular how is the "jump" address to bootloader/kernel set ?)

Thank you very much.

Lorenzo

> 
> Signed-off-by: Doug Anderson <dianders@...omium.org>
> ---
>  arch/arm/mach-exynos/mcpm-exynos.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/mach-exynos/mcpm-exynos.c b/arch/arm/mach-exynos/mcpm-exynos.c
> index 0498d0b..3a7fad0 100644
> --- a/arch/arm/mach-exynos/mcpm-exynos.c
> +++ b/arch/arm/mach-exynos/mcpm-exynos.c
> @@ -343,11 +343,13 @@ static int __init exynos_mcpm_init(void)
>  	pr_info("Exynos MCPM support installed\n");
>  
>  	/*
> -	 * Future entries into the kernel can now go
> -	 * through the cluster entry vectors.
> +	 * U-Boot SPL is hardcoded to jump to the start of ns_sram_base_addr
> +	 * as part of secondary_cpu_start().  Let's redirect it to the
> +	 * mcpm_entry_point().
>  	 */
> -	__raw_writel(virt_to_phys(mcpm_entry_point),
> -			ns_sram_base_addr + MCPM_BOOT_ADDR_OFFSET);
> +	__raw_writel(0xe59f0000, ns_sram_base_addr);     /* ldr r0, [pc, #0] */
> +	__raw_writel(0xe12fff10, ns_sram_base_addr + 4); /* bx  r0 */
> +	__raw_writel(virt_to_phys(mcpm_entry_point), ns_sram_base_addr + 8);
>  
>  	iounmap(ns_sram_base_addr);
>  
> -- 
> 2.0.0.526.g5318336
> 
> --
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