Re: [RFC PATCH] pps: Increase PPS_MAX_SOURCES value.

[Rodolfo Giometti]

On 09/06/23 23:00, Charlie Johnston wrote:
> On 6/9/23 02:30, Rodolfo Giometti wrote:
> > On 08/06/23 00:07, Charlie Johnston wrote:
> > > On 6/7/23 02:33, Rodolfo Giometti wrote:
> > > > On 05/06/23 22:31, Charlie Johnston wrote:
> > > > > For consistency with what ptp uses for minors, this
> > > > > change sets PPS_MAX_SOURCES to MINORMASK + 1.
> > > > >
> > > > > The PPS_MAX_SOURCES value is currently set to 16. In
> > > > > some cases this was not sufficient for a system. For
> > > > > example, a system with multiple (4+) PCIe cards each
> > > > > with 4 PTP-capable ethernet interfaces could run out
> > > > > of the available PPS major:minors if each interface
> > > > > registers a PPS source.
> > > > >
> > > > > Signed-off-by: Charlie Johnston <charlie.johnston@xxxxxx>
> > > > > ---
> > > > >     include/uapi/linux/pps.h | 2 +-
> > > > >     1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >
> > > > > diff --git a/include/uapi/linux/pps.h b/include/uapi/linux/pps.h
> > > > > index 009ebcd8ced5..85f472330da8 100644
> > > > > --- a/include/uapi/linux/pps.h
> > > > > +++ b/include/uapi/linux/pps.h
> > > > > @@ -26,7 +26,7 @@
> > > > >     #include <linux/types.h>
> > > > >       #define PPS_VERSION        "5.3.6"
> > > > > -#define PPS_MAX_SOURCES        16        /* should be enough... */
> > > > > +#define PPS_MAX_SOURCES        (MINORMASK + 1)
> > > > >       /* Implementation note: the logical states ``assert'' and ``clear''
> > > > >      * are implemented in terms of the chip register, i.e. ``assert''
> > > >
> > > > I have just one question: are you sure that it's safe to call idr_alloc(..., 0, (MINORMASK + 1), ...)?
> > > >
> > > > Ciao,
> > > >
> > > > Rodolfo
> > >
> > > Thanks for taking a look!
> > >
> > > My understanding is that idr_alloc(..., start, end, ...) can take any end value up to INT_MAX. It also handles any values <= 0 by treating them as equal to INT_MAX + 1 since the end value is non-inclusive. I can't think of any reason using MINORMASK + 1 here would be an issue since it's much less than the maximum value idr_alloc() allows.
> > >
> > > A number of drivers (e.g. ptp) just explicitly use a start and end value of 0, but I don't think that change would fit here.
> >
> > I see and maybe I should replace the usage of idr_*() with ida_*() as PTP does...
> >
> > However the right-thing(TM) to do here should be dropping PPS_MAX_SOURCES at all!
> >
> > Let me go deeper in this issue. I'm going to produce a patch set in next days. Have you any chances to test it?
> >
> > Ciao,
> >
> > Rodolfo
> MINORMASK
> I'll have to check when the system we used for testing is available again (not easy to find a system with 20+ Ethernet ports) but I'd be happy to test a patch!

Great! Please, let me know.

> I know an increase to PPS_MAX_SOURCES was tested on that system.

I see and it seems that it's safer to set PPS_MAX_SOURCES to MINORMASK... so 
please reproduce your patch with this simple modification, then I'm going to 
produce a patch to drop the PPS_MAX_SOURCES define since it's not needed anymore.

After that you should test all these modifications in order to safely add them 
to Linux.

Ciao,

Rodolfo

--
GNU/Linux Solutions                  e-mail: giometti@xxxxxxxxxxxx
Linux Device Driver                          giometti@xxxxxxxx
Embedded Systems                     phone:  +39 349 2432127
UNIX programming                     skype:  rodolfo.giometti