confused by char dev registration in a gpio driver

[Greg KH]

On Tue, May 03, 2011 at 05:21:11PM -0400, Robert P. J. Day wrote:
> 
>   i'm sure i'm going to embarrass myself here but i was perusing the
> char drivers for nice examples, and i ran across this excerpt in
> pc8736x_gpio.c:
> 
> ===== begin =====
> 
>         if (major) {
>                 devid = MKDEV(major, 0);
>                 rc = register_chrdev_region(devid, PC8736X_GPIO_CT, DEVNAME);
>         } else {
>                 rc = alloc_chrdev_region(&devid, 0, PC8736X_GPIO_CT, DEVNAME);
>                 major = MAJOR(devid);
>         }
> 
>         if (rc < 0) {
>                 dev_err(&pdev->dev, "register-chrdev failed: %d\n", rc);
>                 goto undo_request_region;
>         }
>         if (!major) {
>                 major = rc;
>                 dev_dbg(&pdev->dev, "got dynamic major %d\n", major);
>         }
> 
> ===== end =====
> 
>   i'm good with most of that -- if the (parameter) major is explicit,
> then a dev_t of "devid" is created and register_chrdev_region() is
> used.
> 
>   on the other hand, if major is zero, then alloc_chrdev_region() is
> used for *dynamic* allocation of the major number.  in both cases, the
> return code "rc" is saved and, if it's < 0, we have an error.  and
> that's where the confusion comes in.
> 
>   i always thought both of those routines returned a simple zero to
> indicate success.  but look at those last few lines -- that return
> code is assigned to "major", at which point it's *that* value that's
> printed.  wouldn't that just be zero all the time?
> 
>   and wouldn't it also print that this was a "dynamic" major even if
> the user specified an explicit major number at load time?  this second
> point is more nitpicky, but what about that first point?  wouldn't a
> successful registration always print an allocated major number of
> zero?

I think you are right in that this code is wrong.

The chardev stuff is a mess, I keep meaning for years to clean it up.
Any proposals on a sane interface for this stuff is greatly appreciated.

thanks,

greg k-h