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Date:   Wed, 25 Oct 2017 14:54:46 -0500
From:   Eddie James <eajames@...ux.vnet.ibm.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     jdelvare@...e.com, linux-hwmon@...r.kernel.org,
        linux-kernel@...r.kernel.org, andrew@...id.au,
        "Edward A. James" <eajames@...ibm.com>
Subject: Re: drivers (pmbus): ir35221: Set PMBUS_PAGE before reading id and
 model



On 10/23/2017 05:12 PM, Guenter Roeck wrote:
> On Mon, Oct 23, 2017 at 10:23:02AM -0500, Eddie James wrote:
>>
>> On 10/21/2017 11:10 AM, Guenter Roeck wrote:
>>> On Thu, Oct 19, 2017 at 03:57:08PM -0500, eajames@...ux.vnet.ibm.com wrote:
>>>> From: "Edward A. James" <eajames@...ibm.com>
>>>>
>>>> The MFR_ID and MFR_MODEL, which are manually read before probing the
>>>> pmbus core,  are only valid for the two pages that the ir35221 has
>>>> available. Since we don't know the state of the device when we start
>>>> probing, set the page number first before reading id and model.
>>>>
>>> If the device only has two pages, it is highly unlikely that it is possible
>>> to select another page which is not supported; any attempts to do so should
>>> result in a command error.
>>>
>>> Is this theory or based on an actual problem observed ? If so, it will require
>>> some additional explanation.
>> Hi,
>>
>> Yes, I have observed this with a physical ir35221 device off an i2c bus. It
>> seems that there are some places in the PMBUS core that manage to set the
>> page number to 0xFF due to pmbus_set_page being called with page=-1.
> This would be bug(s) in the PMBus core which need(s) to be fixed.
>
>> pmbus_set_page doesn't check the CML register, so if the i2c op succeeds, it
>> will be a success. I can also set it to anything with i2cset tool without
>> error.
>>
>> I would also like to fix calling pmbus_set_page with -1, and I will send up
>> a patch if/when I have time. But I felt it makes sense to also ensure the
>> page is set correctly in the ir35221 driver, since it's an unknown state at
>> probe time, and this device appears a bit "weird" in that it doesn't return
>> an error if it's an unsupported page.
>>
> With that logic, one could argue that we have to reprogram the entire chip
> because, after all, soneone could have messed it up completely using i2cset.
> And the same would be true for all the chips supported by any driver.

Good point... Consider this patch abandoned. I have sent up a patch for 
pmbus core.

Thanks,
Eddie

>
> I'd rather focus on fixing the PMBus core if it really tries to set page #255.
>
> Guenter
>
>> Thanks,
>> Eddie
>>
>>> Thanks,
>>> Guenter
>>>
>>>> Signed-off-by: Edward A. James <eajames@...ibm.com>
>>>> ---
>>>>   drivers/hwmon/pmbus/ir35221.c | 6 ++++++
>>>>   1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/drivers/hwmon/pmbus/ir35221.c b/drivers/hwmon/pmbus/ir35221.c
>>>> index 8b906b4..9a53930 100644
>>>> --- a/drivers/hwmon/pmbus/ir35221.c
>>>> +++ b/drivers/hwmon/pmbus/ir35221.c
>>>> @@ -267,6 +267,12 @@ static int ir35221_probe(struct i2c_client *client,
>>>>   				| I2C_FUNC_SMBUS_READ_BLOCK_DATA))
>>>>   		return -ENODEV;
>>>> +	ret = i2c_smbus_write_byte_data(client, PMBUS_PAGE, 0);
>>>> +	if (ret < 0) {
>>>> +		dev_err(&client->dev, "Failed to set PMBUS_PAGE\n");
>>>> +		return ret;
>>>> +	}
>>>> +
>>>>   	ret = i2c_smbus_read_block_data(client, PMBUS_MFR_ID, buf);
>>>>   	if (ret < 0) {
>>>>   		dev_err(&client->dev, "Failed to read PMBUS_MFR_ID\n");

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