Re: [PATCH V3 4/6] tty: serial: meson: The UART baud rate calculation is described using the common clock code. Also added S4 chip uart Compatible.

[Yu Tu]

Hi Martin,
	Thank you very much for your reply.
On 2022/1/3 3:36, Martin Blumenstingl wrote:
> [ EXTERNAL EMAIL ]
>
> Hi,
>
> On Sat, Jan 1, 2022 at 2:30 PM Yu Tu <yu.tu@xxxxxxxxxxx> wrote:
> [...]
> > > Interesting, thanks for sharing that u-boot turns these clocks on.
> > > Let's say someone wanted to make u-boot save power and turn off all
> > > UART clocks except the one for uart_AO (where we typically connect the
> > > serial console).
> > > In that case the pclk of uart_C (just to choose an example here) is
> > > turned off. Would there be a problem then accessing the registers of
> > > uart_C before clk_prepare_enable is called?
> > The way you describe it, it does hang. This would not be recommended on
> > actual projects.
> >
> > At present, AmLogic chips are older than S4 Soc, and we have no way to
> > deal with this problem. We have to tell customers not to use it in this
> > way。Customers rarely use it in real projects.On the S4 SOC we will use
> > a clock like the UART pclk to control the shutdown using two registers,
> > one safe (need to operate in EL3) and one normal (EL1). It will only be
> > closed if both registers are closed. This mainly prevents misoperation.
> oh, interesting that there's some updates specifically with the S4 SoCs :-)
>
> > With your experience, I'd like to know how you deal with this kind of
> > problem.
> Before this patch the driver simply turns on the clock from within
> meson_uart_probe() (specifically it does so in
> meson_uart_probe_clock()).
> I think there's advanced power-saving techniques. Maybe for now we
> keep it simple and just enable the clock(s) at probe time and disable
> them at driver remove time. What do you think?
I agree with you.
> Best regards,
> Martin