Re: [PATCH v2 01/10] media: ov772x: allow i2c controllers without I2C_FUNC_PROTOCOL_MANGLING

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On Wed, Apr 18, 2018 at 12:05:49PM +0200, jacopo mondi wrote:
> Hi Akinobu,
> 
> On Mon, Apr 16, 2018 at 11:51:42AM +0900, Akinobu Mita wrote:
> > The ov772x driver only works when the i2c controller have
> > I2C_FUNC_PROTOCOL_MANGLING.  However, many i2c controller drivers don't
> > support it.
> >
> > The reason that the ov772x requires I2C_FUNC_PROTOCOL_MANGLING is that
> > it doesn't support repeated starts.
> >
> > This changes the reading ov772x register method so that it doesn't
> > require I2C_FUNC_PROTOCOL_MANGLING by calling two separated i2c messages.
> >
> > Cc: Jacopo Mondi <jacopo+renesas@xxxxxxxxxx>
> > Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> > Cc: Hans Verkuil <hans.verkuil@xxxxxxxxx>
> > Cc: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> > Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Akinobu Mita <akinobu.mita@xxxxxxxxx>
> > ---
> > * v2
> > - Replace the implementation of ov772x_read() instead of adding an
> >   alternative method
> 
> I now wonder if my initial reply to this patch was wrong, and where
> possible we should try to use smbus operations...
> 
> From Documentation/i2c/smbus-protocol
> "If you write a driver for some I2C device, please try to use the SMBus
> commands if at all possible... " and that's because, according to
> documentation, most I2c adapters support smbus protocol but may not
> support the full i2c command set.
> 
> The fact this driver then restricts the supported adapters to the ones
> that support protocol mangling makes me think your change is fine,
> but as it often happens, I would scale this to more knowledgable
> people...

Do you actually need to use this on SMBus adapters? A lot of sensor drivers
just use I²C; if SMBus support is really needed it can be always added back
later on...

-- 
Sakari Ailus
e-mail: sakari.ailus@xxxxxx
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