linux-kernel - Re: [PATCH 2/5] clocksource: Add Allwinner SoCs HS timers driver

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Date:	Sun, 29 Sep 2013 01:34:08 -0300
From:	Emilio López <emilio@...pez.com.ar>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
CC:	Daniel Lezcano <daniel.lezcano@...aro.org>,
	linux-kernel@...r.kernel.org, kevin.z.m.zh@...il.com,
	sunny@...winnertech.com, shuge@...winnertech.com,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/5] clocksource: Add Allwinner SoCs HS timers driver

Hi Maxime,

El 26/09/13 10:13, Maxime Ripard escribió:
> On Wed, Sep 25, 2013 at 08:23:14PM -0300, Emilio López wrote:
>> El 25/09/13 11:03, Maxime Ripard escribió:
>>> Most of the Allwinner SoCs (at this time, all but the A10) also have a
>>> High Speed timers that are not using the 24MHz oscillator as a source
>>> but rather the AHB clock running much faster.
>>>
>>> The IP is slightly different between the A10s/A13 and the one used in
>>> the A20/A31, since the latter have 4 timers available, while the former
>>> have only 2 of them.
>>>
>>> Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
>>> ---
>>
>> [...]
>>
>>> +static void __init sun5i_timer_init(struct device_node *node)
>>> +{
>>> +	unsigned long rate;
>>> +	struct clk *clk;
>>> +	int ret, irq;
>>> +	u32 val;
>>> +
>>> +	timer_base = of_iomap(node, 0);
>>> +	if (!timer_base)
>>> +		panic("Can't map registers");
>>> +
>>> +	irq = irq_of_parse_and_map(node, 0);
>>> +	if (irq <= 0)
>>> +		panic("Can't parse IRQ");
>>> +
>>> +	clk = of_clk_get(node, 0);
>>> +	if (IS_ERR(clk))
>>> +		panic("Can't get timer clock");
>>
>> I'm not familiar with clocksources, but does this have to be as
>> fatal as it is considering the kernel also supports the slower sun4i
>> timer?
>
> Hmmm, I don't know, one might choose to enable only this timer, in that
> case that would make sense to panic, since it would be the only timer in
> that case.

Fair enough.

>> Also, would any special considerations be needed when adjusting the
>> ahb clock? A future cpufreq driver will most likely need to.
>
> While this will be needed at some point, I don't really see how to
> handle that properly. The clock framework doesn't seem to have any
> callback when it comes to reconfiguring a clock that a device might
> use.

Maybe we should consider using one of the other timers; from a quick 
look at the A20 user manual, it seems they can run at ~200MHz ("PLL6/6" 
as input)

> This will also creates trouble for IPs such as the I2C that have to
> setup internal dividers, and use clk_get_rate to do so.

Not really, because they use APB1 which is not scaled. The manual makes 
it explicit when describing APB1:

"This clock is used for some special module apbclk(twi,uart,
ps2, can, scr). Because these modules need special clock rate
even if the apbclk changed."

Cheers,

Emilio
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