Re: [PATCH] drm: amdkfd: Replace (un)register_chrdev() by (unregister/alloc)_chrdev_region()

[Salah Triki <salah.triki@gmail.com>]

On Wed, Mar 05, 2025 at 07:18:33PM -0500, Felix Kuehling wrote:
> 
> On 2025-03-05 16:08, Salah Triki wrote:
> > Replace (un)register_chrdev() by (unregister/alloc)_chrdev_region() as
> > they are deprecated since kernel 2.6.
> 
> Where is that information coming from? I see __register_chrdev documented in
> the current kernel documentation. I see no indication that it's deprecated:
> https://docs.kernel.org/core-api/kernel-api.html#c.__register_chrdev
> 
In the book "Linux Device Drivers" 3ed of J. Corbet and al. (2005), it
is indicated that using register_chrdev() is the old way to register
char drivers, the new code should not use this interface, it should
instead use the cdev interface. 
> 
> >   alloc_chrdev_region() generates a
> > dev_t value, so replace the kfd_char_dev_major int variable by the
> > kfd_char_dev_id dev_t variable and drop the MKDEV() call. Initialize a
> > cdev structure and add it to the device driver model as register_chrdev()
> > used to do and since alloc_chrdev_region() does not do it. Drop the
> > iminor() call since alloc_chrdev_region() allocates only one minor number.
> > On error and in the module exit function, remove the cdev structure from
> > the device driver model as unregister_chrdev() used to do.
> 
> Sounds complicated. Your patch seems to open-code a bunch of details that
> are neatly hidden inside register_chrdev. Why would I want all that detail
> in my driver? I don't see an obvious advantage.
> 
register_chrdev() registers 256 minor numbers, calling it will result in
calling kmalloc_array(256, sizeof(struct probe), GFP_KERNEL) whereas
calling alloc_chrdev_region() with count parameter equals to 1, which is
the number of minor numbers requested, will result in calling
kmalloc_array(1, sizeof(stuct probe), GFP_KERNEL). 

Best Regards,
Salah Triki