linux-kernel - Re: [PATCH v4 3/5] soc/tegra: pmc: Ensure that clock rates aren't too high

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Date:   Thu, 25 Mar 2021 18:02:45 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Jonathan Hunter <jonathanh@...dia.com>,
        Peter Geis <pgwipeout@...il.com>,
        Matt Merhar <mattmerhar@...tonmail.com>,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/5] soc/tegra: pmc: Ensure that clock rates aren't too
 high

25.03.2021 17:39, Thierry Reding пишет:
> On Tue, Mar 02, 2021 at 03:25:00PM +0300, Dmitry Osipenko wrote:
>> Switch all clocks of a power domain to a safe rate which is suitable
>> for all possible voltages in order to ensure that hardware constraints
>> aren't violated when power domain state toggles.
>>
>> Tested-by: Peter Geis <pgwipeout@...il.com> # Ouya T30
>> Tested-by: Nicolas Chauvet <kwizart@...il.com> # PAZ00 T20 and TK1 T124
>> Tested-by: Matt Merhar <mattmerhar@...tonmail.com> # Ouya T30
>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>> ---
>>  drivers/soc/tegra/pmc.c | 92 ++++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 90 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/soc/tegra/pmc.c b/drivers/soc/tegra/pmc.c
>> index f970b615ee27..a87645fac735 100644
>> --- a/drivers/soc/tegra/pmc.c
>> +++ b/drivers/soc/tegra/pmc.c
>> @@ -237,6 +237,7 @@ struct tegra_powergate {
>>  	unsigned int id;
>>  	struct clk **clks;
>>  	unsigned int num_clks;
>> +	unsigned long *clk_rates;
>>  	struct reset_control *reset;
>>  };
>>  
>> @@ -641,6 +642,57 @@ static int __tegra_powergate_remove_clamping(struct tegra_pmc *pmc,
>>  	return 0;
>>  }
>>  
>> +static int tegra_powergate_prepare_clocks(struct tegra_powergate *pg)
>> +{
>> +	unsigned long safe_rate = 100 * 1000 * 1000;
> 
> This seems a bit arbitrary. Where did you come up with that value?
> 
> I'm going to apply this to see how it fares in our testing.

This value is chosen based on the OPP table voltages and frequencies.

Maybe you could add this clarifying comment to the code(?):

"There is no hardware unit on any of Tegra SoCs which requires higher
voltages for the rates above 100MHz."