linux-kernel - Re: [RFC PATCH] pps: Increase PPS_MAX

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Date:   Tue, 20 Jun 2023 15:42:07 -0500
From:   Charlie Johnston <charlie.johnston@...com>
To:     Rodolfo Giometti <giometti@...eenne.com>
Cc:     linux-kernel@...r.kernel.org, brenda.streiff@...com
Subject: Re: [RFC PATCH] pps: Increase PPS_MAX_SOURCES value.

On 6/12/23 11:07, Rodolfo Giometti wrote:
> 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@...com>
>>>>>> ---
>>>>>>     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
> 

I've resubmitted the patch with just PPS_MAX_SOURCES = MINORMASK. The system which hits the limit and causes the problem is currently available for testing. 

Is there anything you'd like me to try running? Or just confirm the limit change works?

Thanks,
Charlie