linux-kernel - Re: [PATCH v2] regulator: core: repeat voltage setting request for stepped regulators

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Message-ID: <354b1edf-479d-469a-931f-e0ef0833ad2c@nvidia.com>
Date: Tue, 29 Jul 2025 10:18:22 +0100
From: Jon Hunter <jonathanh@...dia.com>
To: Romain Gantois <romain.gantois@...tlin.com>,
 Liam Girdwood <lgirdwood@...il.com>, Mark Brown <broonie@...nel.org>
Cc: Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 linux-kernel@...r.kernel.org,
 "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH v2] regulator: core: repeat voltage setting request for
 stepped regulators



On 29/07/2025 10:07, Romain Gantois wrote:
> Hi Jon,
> 
> On Tuesday, 29 July 2025 10:28:17 CEST Jon Hunter wrote:
>> Hi Romain,
>>
> ...
>> Looking at better closer at the issue, I noticed that it is the
>> 'tps62361-vout' regulator that change is causing problem for. On boot
>> I see regulator_set_voltage_unlocked() called for this regulator and
>> min/max voltage requested is ...
>>
>>    regulator regulator.5: min_uV 1000000 max_uV 1350000
>>
>> The min delta is 300000, but in this case the delta never reaches 0
>> and in fact never converges at all and so remains at 300000.
>>
>> Looking at the above, if the delta never changes, then we get stuck
>> in the above loop forever because 'new_delta - delta' is always 0
>> and this is never greater than 'rdev->constraints->max_uV_step'.
>>
>> There are two things that is not clear to me in the above change ...
>>
>> 1. Why do we 'new_delta - delta' instead of 'delta - new_delta'?
>>      Assuming that we should converge, then I would expect that
>>      'new_delta' should be getting smaller as we converge.
> 
> Indeed it should. "new_delta - delta" is equal to the increase of voltage
> "error". So if this value is positive, it's bad because it means we're
> getting further away from the target voltage. Also, if it's negative but
> too large, then it means that we're slowly crawling to the target voltage,
> which is bad. Currently we do:
> 
> ```
> if (new_delta - delta > max_uV_step)
> 	give up and return -EWOULDBLOCK
> ```
> 
> but we should be doing:
> 
> ```
> if (new_delta - delta > -max_uV_step)
> 	give up and return -EWOULDBLOCK
> ```
> 
> which is equivalent to:
> 
> ```
> if (delta - new_delta < max_uV_step)
> 	give up and return -EWOULDBLOCK
> ```
> 
>> 2. If difference in the delta is greater than then 'max_uV_step'
>>      doesn't this imply that we are converging quickly?
>>
> 
> Yes, the current logic is indeed flawed.
> 
>> I am wondering if we need something like ...
>>
>> diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
>> index 8ed9b96518cf..554d83c4af0c 100644
>> --- a/drivers/regulator/core.c
>> +++ b/drivers/regulator/core.c
>> @@ -3884,7 +3884,7 @@ static int regulator_set_voltage_unlocked(struct
>> regulator *regulator, new_delta = ret;
>>
>>                           /* check that voltage is converging quickly enough */
>>   -                       if (new_delta - delta > rdev->constraints->max_uV_step) {
>> +                       if (delta - new_delta < rdev->constraints->max_uV_step) {
> 
> Yes, that would be correct. Do you want to send the fix yourself, or should I
> do it and include your "Suggested-by"?

Given that you more familiar with the logic, please go ahead and send a 
fix. I will happily give it a test on my side.

> Thanks for reporting the issue and sorry for the trouble.

No problem. Thanks for the quick response.

Jon

-- 
nvpublic