linux-kernel - Re: [alsa-devel] [PATCH 15/27] ALSA: hda - Use timecounter

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Date:   Tue, 2 Jan 2018 13:53:01 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     alsa-devel@...a-project.org, Takashi Iwai <tiwai@...e.de>,
        linux-kernel@...r.kernel.org, Vinod Koul <vinod.koul@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sagar Arun Kamble <sagar.a.kamble@...el.com>
Subject: Re: [alsa-devel] [PATCH 15/27] ALSA: hda - Use timecounter_initialize
 interface

On 1/2/18 12:21 PM, Richard Cochran wrote:
> On Tue, Jan 02, 2018 at 11:15:45AM -0600, Pierre-Louis Bossart wrote:
>> I wrote the code for HDaudio and I remember wasting time trying to figure
>> out the gory details of the cycle counter stuff when all I wanted was a
>> conversion from a 24MHz counter to ns values using a 125/3 operation in the
>> right order - as explained in the comments
> 
> Would using clocks_calc_mult_shift() work for you?

In theory yes, but I'd need to re-check what the results would be.
I remember applying the 1/3 factor separately to avoid wrap-around after 
4 hours [1], but I can't remember the details on the analysis. I can't 
figure out what the 'maxsec' argument should be either.

[1] 
http://elixir.free-electrons.com/linux/latest/source/sound/hda/hdac_stream.c#L486