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

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Date:   Mon, 13 Apr 2020 19:52:11 +0530
From:   Rajendra Nayak <rnayak@...eaurora.org>
To:     Jun Nie <jun.nie@...aro.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>, sboyd@...nel.org,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        agross@...nel.org, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Akash Asthana <akashast@...eaurora.org>,
        linux-serial@...r.kernel.org, Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH 02/21] tty: serial: qcom_geni_serial: Use OPP API to set
 clk/perf state



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.

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