[PATCH 7/8] i2c: add 'transferred' field to struct i2c_msg

[Russell King - ARM Linux]

On Thu, Oct 25, 2012 at 03:42:02PM +0200, Jean Delvare wrote:
> On Thu, 25 Oct 2012 14:14:59 +0100, Russell King - ARM Linux wrote:
> > On Thu, Oct 25, 2012 at 02:57:48PM +0200, Jean Delvare wrote:
> > > Hi Felipe, Shubhrajyoti,
> > > 
> > > On Mon, 22 Oct 2012 12:46:57 +0300, Felipe Balbi wrote:
> > > > From: Shubhrajyoti D <shubhrajyoti at ti.com>
> > > > 
> > > > In case of a NACK, it's wise to tell our clients
> > > > drivers about how many bytes were actually transferred.
> > > > 
> > > > Support this by adding an extra field to the struct
> > > > i2c_msg which gets incremented the amount of bytes
> > > > actually transferred.
> > > > 
> > > > Signed-off-by: Shubhrajyoti D <shubhrajyoti at ti.com>
> > > > Signed-off-by: Felipe Balbi <balbi at ti.com>
> > > > ---
> > > >  include/uapi/linux/i2c.h | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > > 
> > > > diff --git a/include/uapi/linux/i2c.h b/include/uapi/linux/i2c.h
> > > > index 0e949cb..4b35c9b 100644
> > > > --- a/include/uapi/linux/i2c.h
> > > > +++ b/include/uapi/linux/i2c.h
> > > > @@ -77,6 +77,7 @@ struct i2c_msg {
> > > >  #define I2C_M_NO_RD_ACK		0x0800	/* if I2C_FUNC_PROTOCOL_MANGLING */
> > > >  #define I2C_M_RECV_LEN		0x0400	/* length will be first received byte */
> > > >  	__u16 len;		/* msg length				*/
> > > > +	__u16 transferred;	/* actual bytes transferred             */
> > > >  	__u8 *buf;		/* pointer to msg data			*/
> > > >  };
> > > 
> > > On the principle I am fine with this, however you also need to define
> > > who should initialize this field, and to what value.
> > 
> > You also miss one very very very big point.  This will break every I2C
> > using userspace program out there unless it is rebuilt - this change will
> > require the exact right version of those userspace programs for the
> > kernel that they're being used on.
> 
> How that? The extra field is added in a hole, so we don't change the
> struct size nor the relative positions of existing fields. Why would
> user-space care?

You know the layout of that struct for certain across all Linux supported
architectures, including some of the more obscure ones which may not
require pointers to be aligned?