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Date:   Mon, 12 Jun 2023 18:07:52 +0200
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Subject: Re: [RFC PATCH] pps: Increase PPS_MAX_SOURCES value.
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To:     Charlie Johnston <charlie.johnston@ni.com>
Cc:     linux-kernel@vger.kernel.org, brenda.streiff@ni.com
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From:   Rodolfo Giometti <giometti@enneenne.com>
In-Reply-To: <38f38f66-0fd5-4e6c-4839-8272cef77046@ni.com>
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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@ni.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

-- 
GNU/Linux Solutions                  e-mail: giometti@enneenne.com
Linux Device Driver                          giometti@linux.it
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