Dear Stephen,
In message 51D45F4D.2010908@wwwdotorg.org you wrote:
get_timer() is used by a number of network related services. For information, just grep for it in the net/ and drivers/net/ directories. The "get_timer(0)" used in your code resets a global resource, and has thus the potential of messing up a number of timeouts running elsewhere in the network code. I wonder to which extend this has actually been considered (and tested) ?
I recall you mentioning this before, but can you expand on this a bit please?
For the two platforms I'm familiar with (Tegra and BCM2835), the implementation of get_timer() is simply:
unlong get_timer(ulong base) { ulong time = read_hw_register() time -= base; return time; }
There's no global state involved. Is this implementation of get_timer() wrong somehow? I'm having a hard time envisaging what kind of global state it's supposed to maintain.
It appears you are (basically) right. Eventually I remember an old implementation that has been fixed since.
I checked all implementations I could find (all 102 of them) and they all behave as you showed in your example, i. e. harmless.
An exception is "arch/arm/cpu/sa1100/timer.c" which does not respect the "base" argument at all, i. e. which is broken.
I always thought that every user of get_timer() was supposed to do something like:
ulong base = get_timer(0); ... work to be timed ulong time_diff = get_timer(base);
in other words, every user maintains their own base variable, and hence get_timer(0) doesn't affect any other users.
Yes, you are right. Sorry for causing confusion here.
Best regards,
Wolfgang Denk