Re: [RFCv2 API PATCH 05/28] DocBook: bus_info can no longer be empty.

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Hans,

On Thursday 13 September 2012 12:40:07 Hans Verkuil wrote:
> On Thu 13 September 2012 03:24:53 Laurent Pinchart wrote:
> > On Friday 07 September 2012 15:29:05 Hans Verkuil wrote:
> > > From: Hans Verkuil <hans.verkuil@cisco.com>
> > > 
> > > During the 2012 Media Workshop it was decided that bus_info as returned
> > > by VIDIOC_QUERYCAP can no longer be empty. It should be a unique
> > > identifier, and empty strings are obviously not unique.
> > > 
> > > Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
> > > ---
> > > 
> > >  Documentation/DocBook/media/v4l/vidioc-querycap.xml |   14
> > >  ++++++++++----
> > >  1 file changed, 10 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/Documentation/DocBook/media/v4l/vidioc-querycap.xml
> > > b/Documentation/DocBook/media/v4l/vidioc-querycap.xml index
> > > f33dd74..d5b1248 100644
> > > --- a/Documentation/DocBook/media/v4l/vidioc-querycap.xml
> > > +++ b/Documentation/DocBook/media/v4l/vidioc-querycap.xml
> > > @@ -90,11 +90,17 @@ ambiguities.</entry>
> > > 
> > >  	    <entry>__u8</entry>
> > >  	    <entry><structfield>bus_info</structfield>[32]</entry>
> > >  	    <entry>Location of the device in the system, a
> > > 
> > > -NUL-terminated ASCII string. For example: "PCI Slot 4". This
> > > +NUL-terminated ASCII string. For example: "PCI:0000:05:06.0". This
> > > 
> > >  information is intended for users, to distinguish multiple
> > > 
> > > -identical devices. If no such information is available the field may
> > > -simply count the devices controlled by the driver, or contain the
> > > -empty string (<structfield>bus_info</structfield>[0] = 0).<!-- XXX
> > > pci_dev->slot_name example --></entry>
> > > +identical devices. If no such information is available the field must
> > > +simply count the devices controlled by the driver ("vivi-000"). The
> > > bus_info
> > > +must start with "PCI:" for PCI boards, "PCIe:" for PCI Express boards,
> > > +"usb-" for USB devices, "I2C:" for i2c devices, "ISA:" for ISA devices
> > > and
> > > +"parport" for parallel port devices.
> > 
> > What about being a bit more precise than that ? We could specify what API
> > drivers must use to fill the bus_info field. For instance, for USB
> > devices,
> > usb_make_path() is currently used by most drivers (which, by the way,
> > doesn't return a string that starts with "USB:").
> 
> I thought about that, but should that be defined in the spec? I'm not sure
> if that's the right place.

On the other hand, if we don't specify the format of the bus_info field 
precisely, it will only be usable as an opaque identifier to userspace. What 
do we want to do with bus_info ? Telling otherwise identical devices apart is 
a must, but do we want to provide more information to userspace ? If the field 
had been longer a sysfs path might have been a good idea, but it won't fit.

-- 
Regards,

Laurent Pinchart