linux-kernel - Re: [PATCH v2 2/2] serial: 8250_of: add support for an optional bus clock

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Message-ID: <04facbe3-cd40-4d79-a204-2b91880da331@riscstar.com>
Date: Wed, 9 Apr 2025 17:47:30 -0500
From: Alex Elder <elder@...cstar.com>
To: Yixun Lan <dlan@...too.org>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, benjamin.larsson@...exis.eu,
 bastien.curutchet@...tlin.com, andriy.shevchenko@...ux.intel.com,
 u.kleine-koenig@...libre.com, lkundrak@...sk, devicetree@...r.kernel.org,
 linux-serial@...r.kernel.org, spacemit@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] serial: 8250_of: add support for an optional bus
 clock

On 4/9/25 4:43 PM, Yixun Lan wrote:
> Hi Alex,
> 
> On 14:22 Wed 09 Apr     , Alex Elder wrote:
>> The SpacemiT UART requires a bus clock to be enabled, in addition to
>> it's "normal" core clock.  Look up the optional bus clock by name,
>> and if that's found, look up the core clock using the name "core".
>>
>> Supplying a bus clock is optional.  If no bus clock is needed, the
>> the first/only clock is used for the core clock.
>>
>> Signed-off-by: Alex Elder <elder@...cstar.com>
>> ---
>> v2: Update logic to more check for the optional bus clock first
>>
>>   drivers/tty/serial/8250/8250_of.c | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/serial/8250/8250_of.c b/drivers/tty/serial/8250/8250_of.c
>> index 11c860ea80f60..a90a5462aa72a 100644
>> --- a/drivers/tty/serial/8250/8250_of.c
>> +++ b/drivers/tty/serial/8250/8250_of.c
>> @@ -123,7 +123,16 @@ static int of_platform_serial_setup(struct platform_device *ofdev,
>>   
>>   	/* Get clk rate through clk driver if present */
>>   	if (!port->uartclk) {
>> -		info->clk = devm_clk_get_enabled(dev, NULL);
>> +		struct clk *bus_clk;
> we also need to handle clk in suspend/resume procedure, so
> I think you need to put bus_clk inside struct of_serial_info..

OK, I didn't do anything for that in previous versions of the
series.

I think that means we'd call clk_disable_unprepare() on
the bus clock after doing so for the function clock.  And
clk_prepare_enable() on the bus clock before doing that for
the function clock in of_serial_resume().  That's easy.

Is there anything further you think is required?  There is
no clock rate associated with the bus clock that I know of,
so even if the function clock rate changes, the bus clock
can remain as-is.

> 
>> +
>> +		bus_clk = devm_clk_get_optional_enabled(dev, "bus");
> for the 'optional', we can interpret it's optional for other vendor
> UART, but a must required clk for SpacemiT's k1 UART controller
> 
> would it better to guard this inside a compatible test or even introduce
> a flag in compatible data?

I don't personally think so. We could, but the DT binding is going
out of its way to define when the bus clock is required.  This is
simpler, and generic.

					-Alex

> 	if (of_device_is_compatible(pdev->dev.of_node, "spacemit,k1-uart")) {
> 		bus_clk = devm_clk_get_enabled(dev, "bus");
> 		..
> 	}
> 
> }
>> +		if (IS_ERR(bus_clk)) {
>> +			ret = dev_err_probe(dev, PTR_ERR(bus_clk), "failed to get bus clock\n");
>> +			goto err_pmruntime;
>> +		}
>> +
>> +		/* If the bus clock is required, core clock must be named */
>> +		info->clk = devm_clk_get_enabled(dev, bus_clk ? "core" : NULL);
>>   		if (IS_ERR(info->clk)) {
>>   			ret = dev_err_probe(dev, PTR_ERR(info->clk), "failed to get clock\n");
>>   			goto err_pmruntime;
>> -- 
>> 2.45.2
>>
>