Re: RFC: PCI SD host controller driver & mmc/mmcsd modules improvements

M. Warner Losh wrote:
> In message: <48F7121A.2010307_at_FreeBSD.org>
>             Alexander Motin <mav_at_freebsd.org> writes:
> : Stanislav Sedov wrote:
> : > On Wed, 15 Oct 2008 23:25:11 +0300
> : > Alexander Motin <mav_at_FreeBSD.org> mentioned:
> : >> Completely fortunate I have noticed that number of iterations depends on 
> : >> my laptop power source. After small investigation I have found that it 
> : >> actually depends on dev.cpu.0.freq value. With default value 2400 I have 
> : >> only several iterations. But every double frequency decrease doubles 
> : >> iteration count. With minimum value 100MHz I have more then 100 
> : >> iterations. Same time it doesn't looks like this time is a real wall 
> : >> time. It looks like DELAY() used in a loop has some problems with time 
> : >> counting.
> : > 
> : > What do you mean? DELAY(9) on your laptop doesn't correspond to the
> : > real time? 
> : 
> : Yes. It works fine when laptop operates at full frequency, but
> : proportionally reduces time interval when powerd drops frequency down. I
> : have also evidence about the same problem on another laptop with
> : 7.1-PRERELEASE.
> 
> Is the slower clock making DELAY take less/more time?  Or is the
> slower clock fed to the SDHCI part who feeds it to the SD card so less
> time accumulates on the SD card because the clock line to it is
> running slower?
> 
> : > AFAIK, DELAY(9) relies on current timecounter for time
> : > accountiong, so there might be problems with it. Have you tried
> : > switching the kern.timecounter.hardware sysctl to see if it will
> : > affect results?
> : 
> : It was late and I am not very aware in FreeBSD time counting, so I have
> : not tried to investigate it deeper.
> 
> I would have thought that if DELAY(10) went from 10us to 100us because
> you are battery power, you'd have more cards working rather than
> fewer..

No, it's opposite. With lower frequency I have proportionally smaller
delays (more loop iterations). I don't remember exact numbers now, but
general tendency was like: with 2400MHz - 10 iterations, with 1200MHz -
20 iterations and with 100MHz - 240 iterations. But neither syslog, nor
my eyes saw any visible delay there.

It looks like working on battery power DELAY() code expects timer speed
reduced, while estimating final timer value, but looks like timer itself
runs on full speed.

-- 
Alexander Motin