Query regarding alloc_chrdev_region()

Query regarding alloc_chrdev_region()

[Rajat Jain]

Hi,

I was wondering that while using dynamic device numner allocation, can
we ever have a scenario like following (using alloc_chrdev_region):

1) Char Driver A is hooked on Major num X, minor numbers 0-10
2) Char Driver B is hooked on (same) Major num X, minor numbers 11-20

If yes, how will the userland get to know about the dynamically
allocated range (needed to create device nodes), since /proc/devices
only shows major numbers?

Thanks,

Rajat
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Re: Query regarding alloc_chrdev_region()

[Greg KH]

On Thu, Mar 29, 2007 at 10:50:34PM +0530, Rajat Jain wrote:
> Hi,
> 
> I was wondering that while using dynamic device numner allocation, can
> we ever have a scenario like following (using alloc_chrdev_region):
> 
> 1) Char Driver A is hooked on Major num X, minor numbers 0-10
> 2) Char Driver B is hooked on (same) Major num X, minor numbers 11-20
> 
> If yes, how will the userland get to know about the dynamically
> allocated range (needed to create device nodes), since /proc/devices
> only shows major numbers?

Through the exporting of this information in sysfs by using the
device_create() call that you are required to do in order to have your
device show up properly there :)

thanks,

greg k-h
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Re: Query regarding alloc_chrdev_region()

[Rajat Jain]

Hi,

> > I was wondering that while using dynamic device numner allocation, can
> > we ever have a scenario like following (using alloc_chrdev_region):
> >
> > 1) Char Driver A is hooked on Major num X, minor numbers 0-10
> > 2) Char Driver B is hooked on (same) Major num X, minor numbers 11-20

So the above is actually possible?

> >
> > If yes, how will the userland get to know about the dynamically
> > allocated range (needed to create device nodes), since /proc/devices
> > only shows major numbers?
>
> Through the exporting of this information in sysfs by using the
> device_create() call that you are required to do in order to have your
> device show up properly there :)

Is that taken care by cdev_add() itself?

Thanks,

Rajat
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Re: Query regarding alloc_chrdev_region()

[Greg KH]

On Fri, Mar 30, 2007 at 08:06:23AM +0530, Rajat Jain wrote:
> Hi,
> 
> >> I was wondering that while using dynamic device numner allocation, can
> >> we ever have a scenario like following (using alloc_chrdev_region):
> >>
> >> 1) Char Driver A is hooked on Major num X, minor numbers 0-10
> >> 2) Char Driver B is hooked on (same) Major num X, minor numbers 11-20
> 
> So the above is actually possible?

Yes, once you have a major number, you can separate out the minor
numbers how ever you wish to.

> >> If yes, how will the userland get to know about the dynamically
> >> allocated range (needed to create device nodes), since /proc/devices
> >> only shows major numbers?
> >
> >Through the exporting of this information in sysfs by using the
> >device_create() call that you are required to do in order to have your
> >device show up properly there :)
> 
> Is that taken care by cdev_add() itself?

Nope, not at all.

thanks,

greg k-h
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Re: Query regarding alloc_chrdev_region()

[Rajat Jain]

> > >> I was wondering that while using dynamic device numner allocation, can
> > >> we ever have a scenario like following (using alloc_chrdev_region):
> > >>
> > >> 1) Char Driver A is hooked on Major num X, minor numbers 0-10
> > >> 2) Char Driver B is hooked on (same) Major num X, minor numbers 11-20
> >
> > So the above is actually possible?
>
> Yes, once you have a major number, you can separate out the minor
> numbers how ever you wish to.

Hi Greg,

Ok, I understand that we can get a major number from the kernel and
split it into a range of minor numbers.

But does alloc_chrdev_region() reuse the major numbers in such a way?

In other words, is the following scenario possible:

Driver 1
======
alloc_chrdev_region(10 minor nos required)
-> receives major 150 (minor 0-9)

Driver 2
======
alloc_chrdev_region(20 minor nos required)
-> receives major 150 (minor 10-29)

Thanks,

Rajat
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Re: Query regarding alloc_chrdev_region()

[Greg KH]

On Fri, Mar 30, 2007 at 11:56:28AM +0530, Rajat Jain wrote:
> >> >> I was wondering that while using dynamic device numner allocation, can
> >> >> we ever have a scenario like following (using alloc_chrdev_region):
> >> >>
> >> >> 1) Char Driver A is hooked on Major num X, minor numbers 0-10
> >> >> 2) Char Driver B is hooked on (same) Major num X, minor numbers 11-20
> >>
> >> So the above is actually possible?
> >
> >Yes, once you have a major number, you can separate out the minor
> >numbers how ever you wish to.
> 
> Hi Greg,
> 
> Ok, I understand that we can get a major number from the kernel and
> split it into a range of minor numbers.
> 
> But does alloc_chrdev_region() reuse the major numbers in such a way?
> 
> In other words, is the following scenario possible:
> 
> Driver 1
> ======
> alloc_chrdev_region(10 minor nos required)
> -> receives major 150 (minor 0-9)
> 
> Driver 2
> ======
> alloc_chrdev_region(20 minor nos required)
> -> receives major 150 (minor 10-29)

If you look at the code in fs/char_dev.c, this can not happen right now,
however, that could change in the future if we start to see a lot of
different drivers all want dynamic numbers.

In short, don't count on the fact that you will always get a different
major number, it is not guaranteed, and in fact, your driver should
never care about such a thing.

And also userspace doesn't care either, udev handles all of that logic
for you automatically.

Hope this helps,

greg k-h
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