Hi all, 

About this patch , do you have any comments ?  
Thanks for your any comments , corrections and etc. 

Thanks very much!

Followings are last emails :
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-Bob

-----Original Message-----
From: Zhang, Lin-Bao (Linux Kernel R&D)
Sent: 2015年11月16日 18:36
To: 'rjw@rjwysocki.net'; 'viresh.kumar@linaro.org'; 'linux-pm@vger.kernel.org';
'linux-kernel@vger.kernel.org'
Subject: [PATCH v1 0/2] fix the defect that pcc-cpufreq's pcc_get_freq()
sometimes can't get correct CPU frequency


Summary :

These 2 patches are developed for drivers/cpufreq/pcc-cpufreq.c's
pcc_get_freq() function.
The pcc-cpufreq driver's function pcc_get_freq() sometimes returns invalid
frequency information.
Without the these 2 patches , it can be reproduced on booting Linux Kernel with
loading pcc-cpufreq.ko and loop unloading/loading pcc-cpufreq.ko.
This problem has been reported on ProLiant systems, primarily 4-socket systems,
for example DL580Gen8.

Generally speaking, as PCC spec,patch 1 can ideally ensure to get correct cpu
freqency if FW works well.
if it fails, patch 2 can be run to retry 2 times to get cpu freq as workaround.

These 2 patches are written by HP's greg.pearson@hpe.com , enhanced/verified
by Linbao.zhang@hpe.com

PATCH 1/2:
in Processor Clocking Control specification v1.0 ,there are 2 limitations :
2.1.8 Minimum Time Between Commands
2.1.9 Maximum Time Between Commands
that means 2 commands' interval should be located between Minimum Time
Between Commands and Maximum

Time Between Commands.
this patch implements this feature,while old pcc-cpufreq.c didn't strictly follow
the spec.
ideally , this patch ensure pcc_get_freq() return valid freq based on the fact FW
is working well.

PATCH 2/2:
if pcc_get_freq() returns 0, we will retry 2 times at most. This is a workaround
patch, although it is 100% perfect.This works around a firmware/timing issue. In
very rare case , it also will fail after retrying 2 times due to firmware issue. Based
on current pcc cpufreq design , this workaround would be the best solution by
confirmation with FW guys. patch 2 has been adoted by sles11sp4.

Detailed testing data can be found as following :

our testing result on DL580Gen8 with upstream kernel:
a) loop unloading/loading pcc driver within 14 hours , pcc_get_freq() always
succeed , the patch always succeeded, don't need the second patch to
workaround
b) loop rebooting DL580Gen8 with git kernel: by writting a shell script, found
about rebooting 200+ times, patch 1 and patch 2 may both failed once, means
pcc_get_freq() returns 0 frequency,although its possibility is very very low.
pcc_get_freq() doesn't affect pcc-cpufreq driver loading ,because they are
running different path.
in this loop rebooting test , pcc-cpufreq can always be loaded successfully.

We can say ,patch 1 and patch 2 would be the best solution for this issue.

Zhang Lin-Bao (2):
  enforce max/min time between commands
  retry 2 times when getting cpu frequency is zero

 drivers/cpufreq/pcc-cpufreq.c | 51
++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

--
1.8.5.2