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

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Message-ID: <b794bccc-0233-4d78-df6f-bf7c688a7d7d@ni.com>
Date:   Wed, 7 Jun 2023 17:07:39 -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 6/7/23 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.

Regards,
Charlie