linux-kernel - Re: [patch 05/26] posix-clocks: Remove interval timer facility and mmap/fasync callbacks

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Date:   Wed, 31 May 2017 11:00:21 +0200
From:   Richard Cochran <richardcochran@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [patch 05/26] posix-clocks: Remove interval timer facility and
 mmap/fasync callbacks

On Tue, May 30, 2017 at 11:15:38PM +0200, Thomas Gleixner wrote:
> The only user of this facility is ptp_clock, which does not implement any of
> those functions.
> 
> Remove them to prevent accidental users. Especially the interval timer
> interfaces are now more or less impossible to implement because the
> necessary infrastructure has been confined to the core code. Aside of that
> it's really complex to make these callbacks implemented according to spec
> as the alarm timer implementation demonstrates. If at all then a nanosleep
> callback might be a reasonable extension. For now keep just what ptp_clock
> needs.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Acked-by: Richard Cochran <richardcochran@...il.com>

For the record, recently someone did try to implement this interface
for the i210 card.  Although there were issues in the implementation,
still I was curious enough to test the performance.

   https://www.mail-archive.com/linuxptp-devel@lists.sourceforge.net/msg01738.html

The result confirmed my expectation that using the normal system clock
sychronized to the PHC using the phc2sys utility yields better
performance than the direct timer_settime() implementation.

The nanosleep() idea might be worthwhile, but only for devices with
directly mapped registers (like in some SoCs) and not for the common
PCIe devices.  In any case, I am happy to see the timer interface
removed, as it is basically useless and gives people false
impressions.

Could you please include this documentation fix, too?

Thanks,
Richard

---8<---
diff --git a/Documentation/ptp/ptp.txt b/Documentation/ptp/ptp.txt
index ae8fef8..11e904e 100644
--- a/Documentation/ptp/ptp.txt
+++ b/Documentation/ptp/ptp.txt
@@ -18,7 +18,6 @@
     - Adjust clock frequency
 
   + Ancillary clock features
-    - One short or periodic alarms, with signal delivery to user program
     - Time stamp external events
     - Period output signals configurable from user space
     - Synchronization of the Linux system time via the PPS subsystem
@@ -48,9 +47,7 @@
    User space programs may control the clock using standardized
    ioctls. A program may query, enable, configure, and disable the
    ancillary clock features. User space can receive time stamped
-   events via blocking read() and poll(). One shot and periodic
-   signals may be configured via the POSIX timer_settime() system
-   call.
+   events via blocking read() and poll().
 
 ** Writing clock drivers