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

[Guenter Roeck]

On 10/25/2017 11:44 PM, Peter Rosin wrote:
> 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@xxxxxxxxxx>
> > > > > > > ---
> > > > > > >     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.

I would suggest .../bindings/hwmon/jc42.txt. Even though the functionality is
supported by various SMBus chips, it is not supported by _every_ SMBus chip.

I also found this:

https://github.com/opennetworklinux/linux/blob/master/3.2.65-1%2Bdeb7u2/patches/driver-adt7470-knob-to-disable-smbus-timeout.patch

which suggests that we should find a common solution (even though that patch
never found its way upstream).

Rob, are you ok with "smbus-timeout-disable" as suggested above ?

Thanks,
Guenter