Re: [PATCH 3/5] drm/dp: Implement I2C_M_STOP for i2c-over-aux

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Dec 12, 2018 at 11:30:30AM +0100, Daniel Vetter wrote:
> On Mon, Dec 10, 2018 at 06:47:00PM -0800, Dhinakaran Pandiyan wrote:
> > On Fri, 2018-09-28 at 21:04 +0300, Ville Syrjala wrote:
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > 
> > > Consult the I2C_M_STOP flag to determine whether to set the MOT bit
> > > or
> > > not. Makes it possible to send multiple messages in one go with
> > > stop+start generated between the messages (as opposed nothing or
> > > repstart depending on whether thr address/rw changed).
> > > 
> > > Not sure anyone has actual use for this but figured I'd handle it
> > > since I started to look at that flag for MST remote i2c xfers.
> > > 
> > Don't see the I2C_M_STOP flag anywhere in drm_edid.c, but the change
> > introduced here does make sense.
> 
> Iirc it's the i2c core library which takes an entire transaction, splits
> it up, and sets the stop flag only on the very last one. Or something like
> that.

The last msg of the transfer has an implicit stop even without
the flag. The core won't add the flag for you. So the flag
is purely meant to force a stop+start between two messages
of the same transfer.

Well, it's not really specific anywhere IIRC but that's how
i2c-algo-bit behaves, and I tend to think of that one as the
defacto specification.


> -Daniel
> 
> > 
> > Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
> > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/drm_dp_helper.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_dp_helper.c
> > > b/drivers/gpu/drm/drm_dp_helper.c
> > > index 37c01b6076ec..e85cea299d2a 100644
> > > --- a/drivers/gpu/drm/drm_dp_helper.c
> > > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > > @@ -884,7 +884,8 @@ static void drm_dp_i2c_msg_set_request(struct
> > > drm_dp_aux_msg *msg,
> > >  {
> > >  	msg->request = (i2c_msg->flags & I2C_M_RD) ?
> > >  		DP_AUX_I2C_READ : DP_AUX_I2C_WRITE;
> > > -	msg->request |= DP_AUX_I2C_MOT;
> > > +	if (!(i2c_msg->flags & I2C_M_STOP))
> > > +		msg->request |= DP_AUX_I2C_MOT;
> > >  }
> > >  
> > >  /*
> > 
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> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Ville Syrjälä
Intel
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