linux-kernel - Re: [PATCH v2 10/12] watchdog: s3c2410: Support separate source clock

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Message-ID: <b65c74f3-0735-7b8b-888a-3b14aee55a4f@roeck-us.net>
Date:   Sun, 7 Nov 2021 08:09:53 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Sam Protsenko <semen.protsenko@...aro.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Rob Herring <robh+dt@...nel.org>,
        linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org
Subject: Re: [PATCH v2 10/12] watchdog: s3c2410: Support separate source clock

On 11/7/21 7:55 AM, Sam Protsenko wrote:
> On Tue, 2 Nov 2021 at 12:15, Krzysztof Kozlowski
> <krzysztof.kozlowski@...onical.com> wrote:
>>
>> On 31/10/2021 13:22, Sam Protsenko wrote:
>>> Right now all devices supported in the driver have the single clock: it
>>> acts simultaneously as a bus clock (providing register interface
>>> clocking) and source clock (driving watchdog counter). Some newer Exynos
>>> chips, like Exynos850, have two separate clocks for that. In that case
>>> two clocks will be passed to the driver from the resource provider, e.g.
>>> Device Tree. Provide necessary infrastructure to support that case:
>>>    - use source clock's rate for all timer related calculations
>>>    - use bus clock to gate/ungate the register interface
>>>
>>> All devices that use the single clock are kept intact: if only one clock
>>> is passed from Device Tree, it will be used for both purposes as before.
>>>
>>> Signed-off-by: Sam Protsenko <semen.protsenko@...aro.org>
>>> ---
>>> Changes in v2:
>>>    - Reworded commit message to be more formal
>>>    - Used separate "has_src_clk" trait to tell if source clock is present
>>>    - Renamed clock variables to match their purpose
>>>    - Removed caching source clock rate, obtaining it in place each time instead
>>>    - Renamed err labels for more consistency
>>>
>>>   drivers/watchdog/s3c2410_wdt.c | 52 +++++++++++++++++++++++++---------
>>>   1 file changed, 39 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
>>> index fdb1a1e9bd04..c733917c5470 100644
>>> --- a/drivers/watchdog/s3c2410_wdt.c
>>> +++ b/drivers/watchdog/s3c2410_wdt.c
>>> @@ -118,7 +118,9 @@ struct s3c2410_wdt_variant {
>>>
>>>   struct s3c2410_wdt {
>>>        struct device           *dev;
>>> -     struct clk              *clock;
>>> +     struct clk              *bus_clk; /* for register interface (PCLK) */
>>> +     struct clk              *src_clk; /* for WDT counter */
>>> +     bool                    has_src_clk;
>>
>> Why do you need the has_src_clk value? If clk_get() fails, just store
>> there NULL and clk API will handle it.
>>
> 
> I've added that 'has_src_clk' field for somewhat different reason.
> 
> 1. As we discussed earlier, in case when src_clk is not present, I do
> 'src_clk = bus_clk' in v2. This way I don't have to check if src_clk
> is NULL every time and use bus_clk to get rate. If I set src_clk =
> NULL, I'll have to add selector code, which wouldn't be so elegant,
> and contradicts what we've discussed.
> 2. On the other hand, I don't want to enable bus_clk twice in case
> when src_clk is not present, that just doesn't feel right (user would
> see incorrect refcount value in DebugFS, etc). And if "clk_src =
> bus_clk', and I haven't enabled it second time, then I shouldn't try
> to disable it second time, right?
> 
> The only way I can see to accomplish (1) and (2) together, is to use
> that 'has_src_clk' flag. For me it still looks better than enabling
> bus_clk twice, or checking if clk_src is NULL every time we need to
> get clock rate. Please let me know if you still have a strong opinion
> on this one.
> 

This is odd. Why do you need to get the clock rate more than once,
instead of getting it once and storing it locally ? If the clock rate
can change, the watchdog timeout would be unpredictable.

Guenter