[RFC][PATCHv2] new applets: i2c-tools

[Isaac Dunham]

On Fri, Dec 12, 2014 at 12:09:55AM +0100, Xabier Oneca -- xOneca wrote:
> Hello,
> 2014-12-11 23:41 GMT+01:00 tito <farmatito at tiscali.it>:
> > On Thursday 11 December 2014 21:50:56 walter harms wrote:
> >>
> >> Am 11.12.2014 16:36, schrieb Bartosz Golaszewski:
> [...]
> >> > +/* Opens the device file associated with given i2c bus. */
> >> > +static int i2c_dev_open(int i2cbus)
> >> > +{
> >> > +   char filename[32];
> >> > +   int fd;
> >> > +
> >> > +   snprintf(filename, sizeof(filename), "/dev/i2c-%d", i2cbus);
> >> > +   fd = open_or_warn(filename, O_RDWR);
> >>
> >> now user of "/dev/i2c/%d" get warning here even when the next open works.
> >> I do not think that this was you intention.
> >>
> >> /* having one open is more easy :) */
> >>
> >> just my 2 cents,
> >>  wh
> >
> > Hi,
> > I think that this is better than having an error message like this:
> >
> > f (fd < 0)
> >              bb_perror_msg_and_die("Could not open file: /dev/i2c-%d or /dev/i2c/%d", i2cbus, i2cbus);
> >
> > and not knowing exactly which device you was not able to open.
> >
> > Ciao,
> > Tito
> >
> >> > +   if (fd < 0 && errno == ENOENT) {
> >> > +           snprintf(filename, sizeof(filename), "/dev/i2c/%d", i2cbus);
> >> > +           fd = xopen(filename, O_RDWR);
> >> > +   }
> [...]
> 
> If you mistype 'i2cbus' parameter in the command line, you will have
> both a warning and an error opening two devices, which can be
> misleading.
> 
> Why would you even try to open the old-style device in a new kernel,
> or a new-style dev in an old one? I think this can be avoided with the
> usual '#if LINUX_VERSION ...', be it with preprocessor instructions,
> or with a plain 'if ... else' statement.

This is the wrong way, IMHO.
It means that you *cannot* compile busybox with i2ctools that work on
pre-2.6.14(?) kernels, unless you build it using 2.6.14 or older headers;
and in that case, you must recompile busybox if you upgrade the kernel.
It would be tolerable if it were right at the 2.4/2.6 cutoff, but it
isn't.

Also, someone could configure a newer kernel to use the same layout for
/dev as an older kernel in at least two ways:
-if they use a static /dev
-if they configure the hotplug helper to use the old layout (for
example, for backwards compatability).
In mdev, that might look like this:
i2c-([0-9]*)	root:i2c	0660	=i2c/%1

I would support checking both paths.

Thanks,
Isaac Dunham