Re: [PATCH 2/2] ALSA: hda: support for wallclock timestamps

[Takashi Iwai <tiwai@suse.de>]

At Fri, 15 Jun 2012 05:02:38 -0500,
Pierre-Louis Bossart wrote:
> 
> 
> >> +struct azx_timecounter {
> >> +	cycle_t cycle_last;
> >> +	cycle_t mask;
> >> +	cycle_t elapsed_cycles;
> >> +	u64     initial_time_nsec;
> >> +	u32     mult;
> >> +	u32     shift;
> >> +};
> >
> > Any reason not using the normal struct timecounter stuff?
> > Most of the open codes can be replaced gracefully with functions /
> > macros there, I guess.
> 
> Yes there is a reason. The conversion from wall clock cycles to ns is a 
> fractional operation (125/3 ratio from 24 MHz to 1 GHz). If you do this 
> conversion to ns every time, you will accumulate rounding errors. That 
> doesn't seem like a very good design if the precision depends on the 
> duration of the track...

Hm, OK.  It's a shortcoming in the generic timecounter code, IMO.
It might make more sense to fix there.

BTW, the calculation mult can be simplified with
clocksource_khz2multi().

> On top of this, I couldn't find a way to pass the 'chip' argument in the 
> cyclecounter .read() operation to map it to azx_read.

You can use container_of().


Takashi