linux-kernel - 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.

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Date:   Sun, 2 Jan 2022 20:36:48 +0100
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Yu Tu <yu.tu@...ogic.com>
Cc:     linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        Vyacheslav <adeep@...ina.in>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>
Subject: 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.

Hi,

On Sat, Jan 1, 2022 at 2:30 PM Yu Tu <yu.tu@...ogic.com> 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?


Best regards,
Martin