Re: [PATCH 02/21] tty: serial: qcom_geni_serial: Use OPP API to set clk/perf state

From: Jun Nie <jun.nie@linaro.org>
To: Rajendra Nayak <rnayak@codeaurora.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
	sboyd@kernel.org, Bjorn Andersson <bjorn.andersson@linaro.org>,
	agross@kernel.org, linux-arm-msm@vger.kernel.org,
	devicetree@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Akash Asthana <akashast@codeaurora.org>,
	linux-serial@vger.kernel.org,
	Matthias Kaehlcke <mka@chromium.org>
Subject: Re: [PATCH 02/21] tty: serial: qcom_geni_serial: Use OPP API to set clk/perf state
Date: Tue, 14 Apr 2020 13:26:21 +0800	[thread overview]
Message-ID: <CABymUCM+WjSxKhhvqbBSxub_3wxvnJ8aOVgtWD9JOoxA3MuY2A@mail.gmail.com> (raw)
In-Reply-To: <13907000-e3b0-12d6-0768-fd8a7ab100d9@codeaurora.org>

Rajendra Nayak <rnayak@codeaurora.org> 于2020年4月13日周一 下午10:22写道：
>
>
>
> On 4/10/2020 2:06 PM, Jun Nie wrote:
> >>> @@ -961,7 +962,7 @@ static void qcom_geni_serial_set_termios(struct uart_port *uport,
> >>>                goto out_restart_rx;
> >>>
> >>>        uport->uartclk = clk_rate;
> >>> -     clk_set_rate(port->se.clk, clk_rate);
> >>> +     dev_pm_opp_set_rate(uport->dev, clk_rate);
> >
> > Hi Rajendra,
>
> Hi Jun,
>
> > I see lowest rpmhpd_opp_low_svs opp is for 75MHz. It is a bit higher
> > for a serial.
> > I am just curious about this.
>
> Well these OPP tables are technically what we call as fmax tables, which means
> you can get the clock to a max of 75MHz at that perf level. You need to go
> to the next perf level if you want to go higher.
> That however does not mean that serial cannot run at clocks lower than 75Mhz.
>
> > I also want to confirm that the rpmhpd_opp_low_svs voltage restriction
> > is for serial
> > controller, not for clock controller? Because I see there is similar
> > restriction to clock
> > controller on another platform, the restriction is for branch clock,
> > not leaf clock that
> > consumer device will get.
>
> yes, its a serial controller restriction and not of the clock provider.
> On your note on the branch clock vs leaf clock I am not sure I understand
> the point you are making.

For the leaf clock, I mean the clock that consumer get with devm_clk_get(). The
branch clock means it is not for consumer directly, and its child
clock or grandchild
clock is for consumer. In that case, the restriction has to be done in
clock driver,
not in clock consumer driver. Sorry for confusing you. I just want to
know more about
what function this patch set provide. Because I am working on the
clock controller
restriction of fmax/voltage. Thanks!

Jun
>
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