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Date:   Mon, 3 Jul 2023 15:38:42 +0200
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Rodolfo Giometti <giometti@enneenne.com>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] include/uapi pps.h: drop not needed PPS_MAX_SOURCES
 define
Message-ID: <2023070327-gangway-comic-6e15@gregkh>
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On Fri, Jun 30, 2023 at 03:29:50PM +0200, Rodolfo Giometti wrote:
> On 30/06/23 14:53, Greg Kroah-Hartman wrote:
> > On Fri, Jun 30, 2023 at 09:50:33AM +0200, Rodolfo Giometti wrote:
> > > On 30/06/23 09:31, Greg Kroah-Hartman wrote:
> > > > On Fri, Jun 30, 2023 at 09:18:26AM +0200, Rodolfo Giometti wrote:
> > > > > Userspace PPS clients should not known about how many PPS sources can
> > > > > be defined within the system (nor the rfc2783 say so), so we can
> > > > > safely drop this define since is not used anymore in the kernel too.
> > > > > 
> > > > > Signed-off-by: Rodolfo Giometti <giometti@enneenne.com>
> > > > > ---
> > > > >    drivers/pps/pps.c        | 6 +++---
> > > > >    include/uapi/linux/pps.h | 1 -
> > > > >    2 files changed, 3 insertions(+), 4 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> > > > > index 5d19baae6a38..1a6131608036 100644
> > > > > --- a/drivers/pps/pps.c
> > > > > +++ b/drivers/pps/pps.c
> > > > > @@ -354,7 +354,7 @@ int pps_register_cdev(struct pps_device *pps)
> > > > >    	 * Get new ID for the new PPS source.  After idr_alloc() calling
> > > > >    	 * the new source will be freely available into the kernel.
> > > > >    	 */
> > > > > -	err = idr_alloc(&pps_idr, pps, 0, PPS_MAX_SOURCES, GFP_KERNEL);
> > > > > +	err = idr_alloc(&pps_idr, pps, 0, MINORMASK, GFP_KERNEL);
> > > > >    	if (err < 0) {
> > > > >    		if (err == -ENOSPC) {
> > > > >    			pr_err("%s: too many PPS sources in the system\n",
> > > > > @@ -449,7 +449,7 @@ EXPORT_SYMBOL(pps_lookup_dev);
> > > > >    static void __exit pps_exit(void)
> > > > >    {
> > > > >    	class_destroy(pps_class);
> > > > > -	unregister_chrdev_region(pps_devt, PPS_MAX_SOURCES);
> > > > > +	unregister_chrdev_region(pps_devt, MINORMASK);
> > > > >    }
> > > > >    static int __init pps_init(void)
> > > > > @@ -463,7 +463,7 @@ static int __init pps_init(void)
> > > > >    	}
> > > > >    	pps_class->dev_groups = pps_groups;
> > > > > -	err = alloc_chrdev_region(&pps_devt, 0, PPS_MAX_SOURCES, "pps");
> > > > > +	err = alloc_chrdev_region(&pps_devt, 0, MINORMASK, "pps");
> > > > >    	if (err < 0) {
> > > > >    		pr_err("failed to allocate char device region\n");
> > > > >    		goto remove_class;
> > > > > diff --git a/include/uapi/linux/pps.h b/include/uapi/linux/pps.h
> > > > > index 90f2e86020ba..8a4096f18af1 100644
> > > > > --- a/include/uapi/linux/pps.h
> > > > > +++ b/include/uapi/linux/pps.h
> > > > > @@ -26,7 +26,6 @@
> > > > >    #include <linux/types.h>
> > > > >    #define PPS_VERSION		"5.3.6"
> > > > > -#define PPS_MAX_SOURCES		MINORMASK
> > > > 
> > > > Why change this in patch 1, and then delete this here?
> > > > 
> > > > That makes no sense.
> > > 
> > > I did it in two steps to be clear that the first step is about a better
> > > redefinition of the PPS_MAX_SOURCES define, while the second step is about
> > > the fact it's now that define is useless.
> > 
> > Better to just convert everything in patch one, and then in patch 2
> > delete the .h #define.  That way, when userspace breaks, you can revert
> > just the last patch :)
> 
> I'm puzzled since I did as you say... patch 1/2 increases PPS_MAX_SOURCES
> value while patch 2/2 drops PPS_MAX_SOURCES define.

Ah, I thought patch 1/2 renamed it.

But why increase it if you are removing it?

> > > > And if this is exported to userspace, removing it should break things,
> > > > right?  If not, why was it there in the first place?
> > > 
> > > In reality such define is not stated within the PPS RFC2783, so userspace
> > > programs whose relies on such define are broken.
> > 
> > RFC's do not document Linux kernel apis.
> 
> It's true, but well written PPS clients should relay only on PPS API which
> in turn doesn't states that define. :P

Are you sure?  Have you audited the clients?  if so, please document
that in the changelog text.

> > So if any userspace code breaks, you have to put this back, sorry.
> 
> If you think that patch 2 is not good, no problem, just drop it, but please
> just consider applying patch 1, since increasing PPS_MAX_SOURCES value is
> good.

You can't change a uapi value either without breaking things :(

thanks,

greg k-h