linux-kernel - Re: [RFC][PATCH] rtc: sunxi: use external 32k oscillator if provided

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Message-ID: <yw1x8rb1xkwg.fsf@mansr.com>
Date:   Thu, 27 Jul 2023 19:35:27 +0100
From:   Måns Rullgård <mans@...sr.com>
To:     Andre Przywara <andre.przywara@....com>
Cc:     Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        linux-rtc@...r.kernel.org, linux-sunxi@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] rtc: sunxi: use external 32k oscillator if provided

Andre Przywara <andre.przywara@....com> writes:

> Hi Mans,
>
> On 27/07/2023 16:01, Mans Rullgard wrote:
>> Set the OSC32K_SRC_SEL bit in the LOSC control register if a clock is
>> specified in the devicetree.
>> Signed-off-by: Mans Rullgard <mans@...sr.com>
>> ---
>> The newer sun6i rtc driver is a proper clk provider with parent
>> selection.  Doing the same thing in this driver would be difficult
>> while staying compatible with existing devicetrees.  For that reason,
>> this simpler approach seems reasonable.
>> ---
>>   drivers/rtc/rtc-sunxi.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>> diff --git a/drivers/rtc/rtc-sunxi.c b/drivers/rtc/rtc-sunxi.c
>> index 5d019e3a835a..4f1053eab778 100644
>> --- a/drivers/rtc/rtc-sunxi.c
>> +++ b/drivers/rtc/rtc-sunxi.c
>> @@ -5,6 +5,7 @@
>>    * Copyright (c) 2013, Carlo Caione <carlo.caione@...il.com>
>>    */
>>   +#include <linux/clk.h>
>>   #include <linux/delay.h>
>>   #include <linux/err.h>
>>   #include <linux/fs.h>
>> @@ -21,8 +22,10 @@
>>   #include <linux/types.h>
>>     #define SUNXI_LOSC_CTRL				0x0000
>> +#define SUNXI_LOSC_CTRL_KEY			(0x16aa << 16)
>>   #define SUNXI_LOSC_CTRL_RTC_HMS_ACC		BIT(8)
>>   #define SUNXI_LOSC_CTRL_RTC_YMD_ACC		BIT(7)
>> +#define SUNXI_LOSC_CTRL_OSC32K_SRC_SEL		BIT(0)
>>     #define SUNXI_RTC_YMD				0x0004
>>   @@ -422,6 +425,7 @@ MODULE_DEVICE_TABLE(of, sunxi_rtc_dt_ids);
>>   static int sunxi_rtc_probe(struct platform_device *pdev)
>>   {
>>   	struct sunxi_rtc_dev *chip;
>> +	struct clk *extclk;
>>   	int ret;
>>     	chip = devm_kzalloc(&pdev->dev, sizeof(*chip), GFP_KERNEL);
>> @@ -455,6 +459,14 @@ static int sunxi_rtc_probe(struct platform_device *pdev)
>>   		return -ENODEV;
>>   	}
>>   +	/* use external oscillator if present */
>> +	extclk = devm_clk_get_optional_enabled(&pdev->dev, NULL);
>> +	if (IS_ERR(extclk))
>> +		return PTR_ERR(extclk);
>> +	if (extclk)
>> +		writel(SUNXI_LOSC_CTRL_KEY | SUNXI_LOSC_CTRL_OSC32K_SRC_SEL,
>> +		       chip->base + SUNXI_LOSC_CTRL);
>
> This should be a read-modify-write operation, since we don't want to disturb
> other bits in this register.

Good point.  I guess it's best to leave everything untouched if the
clock isn't specified, just in case someone has a bootloader that sets
this bit.

> In general this looks OK to me, but would need to be documented in the
> bindings docs, to allow an optional clocks property.

Sure, I'll make a patch for that as well.

-- 
Måns Rullgård