linux-kernel - Re: [PATCH 1/2] hwmon: (pmbus) Add a NO_PEC flag to probe chips with faulty CAPABILITY

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Message-ID: <7ee04917-d023-8888-9f3f-4303c496f80d@roeck-us.net>
Date:   Mon, 21 Dec 2020 10:53:58 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Eddie James <eajames@...ux.ibm.com>, linux-hwmon@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, jdelvare@...e.com, bjwyman@...il.com
Subject: Re: [PATCH 1/2] hwmon: (pmbus) Add a NO_PEC flag to probe chips with
 faulty CAPABILITY

On 12/21/20 10:32 AM, Eddie James wrote:
> On Mon, 2020-12-21 at 08:54 -0800, Guenter Roeck wrote:
>> On 12/21/20 8:30 AM, Eddie James wrote:
>>> Some PMBus chips don't respond with valid data when reading the
>>> CAPABILITY register. For instance the register may report that the
>>> chip supports PEC when in reality it does not. For such chips, PEC
>>> must not be enabled while probing the chip, so add a flag so that
>>> device drivers can force PEC off.
>>>
>>
>> I think the flag should indicate that the capability register
>> shall not be read/used. That the capability register is currently
>> only used to check for PEC is secondary. We might,for example,
>> start using it to check for alert support or to check the numeric
>> format.
> 
> OK, that makes sense. I'll rename the flag in v2, how does
> PMBUS_NO_CAPABILITY sound?
> 

sgtm

Thanks,
Guenter

> Thanks for the quick reply,
> Eddie
> 
>>
>> Thanks,
>> Guenter
>>
>>> Signed-off-by: Eddie James <eajames@...ux.ibm.com>
>>> ---
>>>  drivers/hwmon/pmbus/pmbus_core.c |  8 +++++---
>>>  include/linux/pmbus.h            | 10 ++++++++++
>>>  2 files changed, 15 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/hwmon/pmbus/pmbus_core.c
>>> b/drivers/hwmon/pmbus/pmbus_core.c
>>> index 192442b3b7a2..3de1657dde35 100644
>>> --- a/drivers/hwmon/pmbus/pmbus_core.c
>>> +++ b/drivers/hwmon/pmbus/pmbus_core.c
>>> @@ -2204,9 +2204,11 @@ static int pmbus_init_common(struct
>>> i2c_client *client, struct pmbus_data *data,
>>>  	}
>>>  
>>>  	/* Enable PEC if the controller supports it */
>>> -	ret = i2c_smbus_read_byte_data(client, PMBUS_CAPABILITY);
>>> -	if (ret >= 0 && (ret & PB_CAPABILITY_ERROR_CHECK))
>>> -		client->flags |= I2C_CLIENT_PEC;
>>> +	if (!(data->flags & PMBUS_NO_PEC)) {
>>> +		ret = i2c_smbus_read_byte_data(client,
>>> PMBUS_CAPABILITY);
>>> +		if (ret >= 0 && (ret & PB_CAPABILITY_ERROR_CHECK))
>>> +			client->flags |= I2C_CLIENT_PEC;
>>> +	}
>>>  
>>>  	/*
>>>  	 * Check if the chip is write protected. If it is, we can not
>>> clear
>>> diff --git a/include/linux/pmbus.h b/include/linux/pmbus.h
>>> index 1ea5bae708a1..9bdc8a581b03 100644
>>> --- a/include/linux/pmbus.h
>>> +++ b/include/linux/pmbus.h
>>> @@ -34,6 +34,16 @@
>>>   */
>>>  #define PMBUS_WRITE_PROTECTED	BIT(1)
>>>  
>>> +/*
>>> + * PMBUS_NO_PEC
>>> + *
>>> + * Some PMBus chips don't respond with valid data when reading the
>>> CAPABILITY
>>> + * register. In this case, the register may report that the chip
>>> supports PEC
>>> + * with bit 7 (PB_CAPABILITY_ERROR_CHECK) when in reality it's not
>>> supported.
>>> + * For such chips, PEC must not be enabled before probing the
>>> chip.
>>> + */
>>> +#define PMBUS_NO_PEC			BIT(2)
>>> +
>>>  struct pmbus_platform_data {
>>>  	u32 flags;		/* Device specific flags */
>>>  
>>>
>