AW: AW: [PATCH 07/12] kern_tc.c: Remove verification of th_generation for pps functions

[Sebastian Huber]

On 07/04/2022 15:16, Sebastian Huber wrote:
> On 07/04/2022 15:10, Gabriel.Moyano at dlr.de wrote:
>>> Which sequence of function calls and timings cases the problem? This 
>>> should be definitely a test case.
>> The generation is updated every time tc_windup() is called. So it is 
>> more or less a race condition when it happens.
> 
> This is not a race condition. The sequence counter is supposed to ensure 
> a consistent state. You can't simply remove this.
> 
> Maybe the PPS support requires that we use two timehands even for the 
> uniprocessor configuration. For this, we have to understand under which 
> conditions and use cases the generation change is an issue.

In kern_tc.c there is this comment, which is a result from a discussion 
on a FreeBSD mailing list:

/*
  * In FreeBSD, the timehands count is a tuning option from two to 16.  The
  * tuning option was added since it is inexpensive and some FreeBSD 
users asked
  * for it to play around.  The default value is two.  One system which 
did not
  * work with two timehands was a system with one processor and a 
specific PPS
  * device.
  *
  * For RTEMS, in uniprocessor configurations, just use one timehand 
since the
  * update is done with interrupt disabled.
  *
  * In SMP configurations, use a fixed set of two timehands until someone
  * reports an issue.
  */

We definitely need a test case for this corner case. Adding a second 
timehand has a size impact. On a 32-bit machine there is sizeof(struct 
timehands) == 120.

-- 
embedded brains GmbH
Herr Sebastian HUBER
Dornierstr. 4
82178 Puchheim
Germany
email: sebastian.huber at embedded-brains.de
phone: +49-89-18 94 741 - 16
fax:   +49-89-18 94 741 - 08

Registergericht: Amtsgericht München
Registernummer: HRB 157899
Vertretungsberechtigte Geschäftsführer: Peter Rasmussen, Thomas Dörfler
Unsere Datenschutzerklärung finden Sie hier:
https://embedded-brains.de/datenschutzerklaerung/