Re: [PATCH 2/8] ARM: tegra: config the polarity of the request of sys clock

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 08/02/2013 01:48 AM, Joseph Lo wrote:
> On Tue, 2013-07-30 at 06:47 +0800, Stephen Warren wrote:
>> On 07/26/2013 03:15 AM, Joseph Lo wrote:
>>> When suspending to LP1 mode, the SYSCLK will be clock gated. And different
>>> board may have different polarity of the request of SYSCLK, this patch
>>> configure the polarity from the DT for the board.
>>
>>> diff --git a/arch/arm/mach-tegra/pmc.c b/arch/arm/mach-tegra/pmc.c
>>
>>> @@ -238,6 +240,20 @@ void tegra_pmc_suspend_init(void)
>>>  	reg = tegra_pmc_readl(PMC_CTRL);
>>>  	reg |= TEGRA_POWER_CPU_PWRREQ_OE;
>>>  	tegra_pmc_writel(reg, PMC_CTRL);
>>> +
>>> +	reg = tegra_pmc_readl(PMC_CTRL);
>>> +
>>> +	if (!pmc_pm_data.sysclkreq_high)
>>> +		reg |= TEGRA_POWER_SYSCLK_POLARITY;
>>> +	else
>>> +		reg &= ~TEGRA_POWER_SYSCLK_POLARITY;
>>> +
>>> +	/* configure the output inverts while the request is tristated */
>>> +	tegra_pmc_writel(reg, PMC_CTRL);
>>
>> I think s/inverts/polarity/ in that comment would make a lot more sense.
>>
> Yes, thanks.
> 
>> Must _OE be disabled for the code to work correctly? If so, should the
>> code explicitly clear _OE first, since who knows what state it was
>> originally in? Can't we just set the new polarity and enable _OE in a
>> single register write?
>>
> The SYSCLK is super clock that was connected to COP subsystem. It can't
> be disabled when system is running. The boot loader had initialized it
> and brought it to kernel. We follow the HW description in DT of the
> polarity of SCLK to re-configure and re-init again. Then the PMC can
> clock gate it when system go into suspend state.

So it sounds like the bootloader has already configured the clock to a
certain polarity. If so, why do we need to reconfigure it again?

Or, is this code not duplicating something the bootloader must have
done, but simply informing some HW in the PMC that's receiving SYSCLK
how the clock is configured elsewhere?