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

[M. Warner Losh]

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..

Warner