Re: confused by char dev registration in a gpio driver

hi Robert, On Tue, May 3, 2011 at 3:21 PM, Robert P. J. Day <rpjday@crashcourse.ca> 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:

Thanks for taking a look, and questioning what looks wrong.

===== 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);        }

so major is set here, either before the if, or as result of the else.

       if (rc < 0) {                dev_err(&pdev->dev, "register-chrdev failed: %d\n", rc);                goto undo_request_region;        }

so when we get here, rc==0, and major is set FWIW, it looks like I poached the above code from LDD3, which explains why it looks right up to here.. Id agree that theres something wrong/unclear below. major = rc looks kinda rotten. Given that the only purpose was to debug-print, and the "dynamic" fact is wrong, I think I'll prep a patch to yank that crap. Unless you beat me to it :-)

       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?

rday

again, thanks. ~jimc