[U-Boot] [PATCH v4 1/1] NET: Improve TFTP booting performance when CONFIG_USB_KEYBOARD

[Wolfgang Denk <wd@denx.de>]

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

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