[rtc-linux] Re: [PATCH] rtc: sunxi: use external oscillator

[Stephan Roslen <stephan@roslen.de>]

On 06.07.2016 22:20, Maxime Ripard wrote:
> On Wed, Jul 06, 2016 at 10:43:00AM +0200, Stephan Roslen wrote:
>> +	writel(loscctrl, chip->base + SUNXI_LOSC_CTRL);
>> +	udelay(100);
>=20
> Why is that udelay needed?

I found that studying the sunxi 3.4 kernel code and considered it a necessa=
ry delay for the hardware setup and update SUNXI_LOSC_CTRL_SRC_SEL bit. My =
assumption was wrong, it seems. At least a few reboots show, that it is lik=
ely to work without.

>=20
>> +
>> +	loscctrl =3D readl(chip->base + SUNXI_LOSC_CTRL);
>> +	if (!(loscctrl & SUNXI_LOSC_CTRL_SRC_SEL)) {
>> +		dev_err(&pdev->dev, "Error: Set LOSC to external failed.\n");
>> +		dev_err(&pdev->dev, "Warning: RTC time will be wrong!\n");
>> +	}
>> +
>=20
> This isn't needed
>=20
> The issue is actually worse than that.
>=20
> That register controls the losc source for the whole clock tree, so it
> will affect every clock in the system.
>=20
> In order to have that correctly propagated, you should register a new
> mux here in the clock framework, and have all the other clocks using
> that mux as a parent.

I agree. Checking the diagram in subsection 1.5.2 of the A20 manual it seem=
s, that LOSC can be a source for clocks like CPU and some SoC busses. So my=
 patch could indeed mess with the whole clock tree.

>=20
> That's going to be tricky, because the clocks usually probe way
> earlier than the RTC driver. So I'm guessing you could do a clock
> driver that maps the registers, register its clock, and then when the
> RTC probes just takes over what has been setup already by the clock
> driver. This also means removing the ability for the RTC to be
> compiled as a module.

I agree again. Though I think, the RTC should still work as a module. The C=
LK driver could provide the regmap MFD style and the RTC driver may access =
it. Or rather the CLK driver should use an MFD, that is provided early, too=
. Eventually even a syscon? Actually the whole bunch described in subsectio=
n 1.9 (including timers, arlarms, rtc and even the watchdog) would have to =
rely on that MFD. Do I miss a point?

Stephan

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