linux-kernel - Re: [PATCH 1/2] hwmon: (jc42) optionally try to disable the SMBUS timeout

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Date:   Thu, 26 Oct 2017 08:44:37 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Guenter Roeck <linux@...ck-us.net>, Rob Herring <robh@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Russell King <linux@...linux.org.uk>,
        Jean Delvare <jdelvare@...e.com>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-hwmon@...r.kernel.org
Subject: Re: [PATCH 1/2] hwmon: (jc42) optionally try to disable the SMBUS
 timeout

On 2017-10-18 04:38, Guenter Roeck wrote:
> On 10/17/2017 03:16 PM, Rob Herring wrote:
>> On Fri, Oct 13, 2017 at 01:35:27PM -0700, Guenter Roeck wrote:
>>> On Fri, Oct 13, 2017 at 04:26:57PM +0200, Peter Rosin wrote:
>>>> On 2017-10-13 15:50, Guenter Roeck wrote:
>>>>> On 10/13/2017 02:27 AM, Peter Rosin wrote:
>>>>>> With a nxp,se97 chip on an atmel sama5d31 board, the I2C adapter driver
>>>>>> is not always capable of avoiding the 25-35 ms timeout as specified by
>>>>>> the SMBUS protocol. This may cause silent corruption of the last bit of
>>>>>> any transfer, e.g. a one is read instead of a zero if the sensor chip
>>>>>> times out. This also affects the eeprom half of the nxp-se97 chip, where
>>>>>> this silent corruption was originally noticed. Other I2C adapters probably
>>>>>> suffer similar issues, e.g. bit-banging comes to mind as risky...
>>>>>>
>>>>>> The SMBUS register in the nxp chip is not a standard Jedec register, but
>>>>>> it is not special to the nxp chips either, at least the atmel chips
>>>>>> have the same mechanism. Therefore, do not special case this on the
>>>>>> manufacturer, it is opt-in via the device property anyway.
>>>>>>
>>>>>> Signed-off-by: Peter Rosin <peda@...ntia.se>
>>>>>> ---
>>>>>>    Documentation/devicetree/bindings/hwmon/jc42.txt |  4 ++++
>>>>>>    drivers/hwmon/jc42.c                             | 20 ++++++++++++++++++++
>>>>>>    2 files changed, 24 insertions(+)
>>>>>>
>>>>>> diff --git a/Documentation/devicetree/bindings/hwmon/jc42.txt b/Documentation/devicetree/bindings/hwmon/jc42.txt
>>>>>> index 07a250498fbb..f569db58f64a 100644
>>>>>> --- a/Documentation/devicetree/bindings/hwmon/jc42.txt
>>>>>> +++ b/Documentation/devicetree/bindings/hwmon/jc42.txt
>>>>>> @@ -34,6 +34,10 @@ Required properties:
>>>>>>    
>>>>>>    - reg: I2C address
>>>>>>    
>>>>>> +Optional properties:
>>>>>> +- smbus-timeout-disable: When set, the smbus timeout function will be disabled.
>>>>>> +			 This is not supported on all chips.
>>
>> Is this only for jc24 devices or could be any smbus device?
>>
> 
> SMBus timeout is a standard SMBus functionality, so I would say any. It is by
> default enabled on an SMBus device (actually it is not just enabled, it is
> mandatory). The ability to disable it comes handy if a SMBus chip is connected
> to an I2C controller which does not (or not necessarily) follow SMBus rules.
> 
> I had seen that problem myself with MAX6697, and STTS751 (and its driver) also
> supports it.

So, is the approach with an optional smbus-timeout-disable property documented
in .../bindings/hwmon/jc42.txt good-to-go or should it be documented in some
common SMBus client-device file? I don't fine any such beast, so I'm unsure
how to proceed in that case.

Cheers,
Peter