[Openpxa-users] Linux udelay() is way off

[Bjørn Forsman]

2011/1/23 Marek Vasut <marek.vasut at gmail.com>:
> On Friday 21 January 2011 18:46:48 Bjørn Forsman wrote:
>> 2011/1/20 Russell King - ARM Linux <linux at arm.linux.org.uk>:
>> > On Thu, Jan 20, 2011 at 08:18:10PM +0100, Bjørn Forsman wrote:
>> >> [   11.425363] cpufreq-core: saving 518144 as reference value for
>> >> loops_per_jiffy; freq is 312000 kHz
>> >> [   11.425523] cpufreq-core: scaling loops_per_jiffy to 1036288 for
>> >> frequency 624000 kHz
>>
>> [snip]
>>
>> > Is it possible to boot at 624MHz and report the resulting lpj?
>> > Alternatively, call calibrate_delay() after the cpufreq switch to
>> > 624MHz, disabling the 'printed' stuff and see what it reports.
>>
>> Now I have an extra calibrate_delay() after cpufreq changes the frequency.
>>
>> First call:
>> Calibrating delay loop... 103.62 BogoMIPS (lpj=518144)
>>
>> Second call:
>> Calibrating delay loop... 622.59 BogoMIPS (lpj=3112960)
>>
>> (udelay() is correct when having that second call.)
>>
>> It seems cpufreq thinks it transitions from 312 to 624 MHz, but in
>> reality it may be transitioning from 104 to 624 MHz (because it now
>> runs 6 times as fast according to lpj). This may explain the udelay()
>> error ratio (~0.32):
>>
>>   104 MHz / 312 MHz = 0.3333..
>>
>> I think this means either the clock speed or the jiffies interval do
>> not match what the kernel thinks.
>>
>> I also did some tests changing the clock speed in the bootloader. It
>> seems like the only thing that works for the kernel is booting with a
>> 104 MHz clock. Any other frequency and cpufreq will do bad things to
>> udelay().
>>
>> Any thoughts?
>>
>> BTW, I failed at locating the jiffies implementation for PXA310. Any
>> pointers?
>
> DAMN.
>
> I cannot replicate this. Bjorn, can you please write me a step-by-step howto for
> morons?
>
> Ah, also, please update your OBM2 and u-boot from openpxa_new branch ;-)

I updated OBM2, U-Boot and the kernel. The issue is now gone and
udelay is correct. Yay!

Below is a diff of two boot logs, one with old bootloaders and one
with new. Notice the difference in kernel lpj and whether it thinks turbo
mode is enabled or not. Both boots are with the same kernel (2.6.38-rc2).

Any idea why new bootloaders cause "write buffer coherency" failure
(see below)? Do we need to worry?

--- boot.log	2011-01-25 17:24:37.000000000 +0100
+++ boot2_openpxa_new.log	2011-01-25 17:24:39.000000000 +0100
@@ -1,8 +1,10 @@
-U-Boot 2010.06-00741-g48e5192-dirty (Dec 09 2010 - 10:13:49)
+OpenPXA OBM 2, booting ... OK
+
+U-Boot 2010.12-02373-ga7a4c08 (Jan 24 2011 - 14:42:56)

 CPU: Marvell PXA31x A2
 Clock: Core: 312MHz ; Turbo: x2
-       [ SMC: 208MHz ; SRAM: 312MHz ; BUS: 312MHz ; DMC: 312MHz ]
+       [ SMC: 208MHz ; SRAM: 312MHz ; BUS: 208MHz ; DMC: 260MHz ]
 DRAM:  128 MiB
 NAND:  512 MiB
 In:    serial
@@ -36,8 +38,8 @@
 Memory policy: ECC disabled, Data cache writeback
 RO Mode clock: 60.00MHz (inactive)
 Run Mode clock: 312.00MHz (*24)
-Turbo Mode clock: 624.00MHz (*2, inactive)
-HSIO bus clock: 312.00MHz
+Turbo Mode clock: 624.00MHz (*2, active)
+HSIO bus clock: 208.00MHz
 Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 32512
 Kernel command line: console=ttyS0,115200 root=/dev/nfs
nfsroot=10.1.11.41:/srv/nfs ip=0
 PID hash table entries: 512 (order: -1, 2048 bytes)
@@ -59,10 +61,10 @@
 NR_IRQS:288 nr_irqs:288 288
 sched_clock: 32 bits at 3250kHz, resolution 307ns, wraps every 1321528ms
 Console: colour dummy device 80x30
-Calibrating delay loop... 103.83 BogoMIPS (lpj=519168)
+Calibrating delay loop... 622.59 BogoMIPS (lpj=3112960)
 pid_max: default: 32768 minimum: 301
 Mount-cache hash table entries: 512
-CPU: Testing write buffer coherency: ok
+CPU: Testing write buffer coherency: failed, enabling work-around
 NET: Registered protocol family 16
 colibri_pxa3xx_init_eth(): no valid serial boot tag found, taking MAC
from device
 bio: create slab <bio-0> at 0

Best regards,
Bjørn Forsman