Subject: Re: [PATCH 1/1] libata: pata_pdc2027x PLL input clock fix

From: Albert Lee <[email-masked]>
Date: Tue, 10 Jul 2007 11:52:59 +0800

Mikael Pettersson wrote:
> (cc:ing linuxppc-dev)
> 
> On Tue, 26 Jun 2007 13:43:15 +0800, Albert Lee wrote:
> 
>>Recently the PLL input clock of pata_pdc2027x is sometimes detected
>>higer than expected (e.g. 20.027 MHz compared to 16.714 MHz).
>>It seems sometimes the mdelay() function is not as precise as it
>>used to be. Per Alan's advice, HT or power management might affect
>>the precision of mdelay().
>>
>>This patch calls gettimeofday() to mesure the time elapsed and
>>calculate the PLL input clock accordingly.
> 
> 
> Unfortunately this breaks pata_pdc2027x on my PowerMac G3:
> 
<snip>
> 
> In fairness, this is a slightly non-standard PowerMac G3, in that it
> has a G4 upgrade processor. The firmware doesn't recognise the CPU
> and gives some CPU core frequency-related properties too low values.
> However, the bus frequency property _is_ correct, which is what
> most or all timing stuff should be based on.
> 
> Looks like a platform bug.
> 

According to the document, do_gettimeofday() has microsecond
resolution. Since the driver calls mdelay(100) (100000 microseconds), 
it won't affect the PLL input clock calculation much if somehow
do_gettimeofday() drifts several (say 100) microseconds.

Could you please apply the attached debug patch and collect more info
on the PowerMac G3. Hopefully we can have more clue. Thanks.
--
albert

(BTW, maybe opening a bug in bugzilla.kernel.org would help the
debugging.)

--- 00_libata-dev/drivers/ata/pata_pdc2027x.c	2007-07-07 09:58:55.000000000 +0800
+++ 01_debug/drivers/ata/pata_pdc2027x.c	2007-07-10 11:18:38.000000000 +0800
@@ -722,6 +722,15 @@ static long pdc_detect_pll_input_clock(s
 	pll_clock = (start_count - end_count) / 100 *
 		(100000000 / usec_elapsed);
 
+	do_gettimeofday(&start_time);
+	mdelay(37);
+	do_gettimeofday(&end_time);
+	usec_elapsed = (end_time.tv_sec - start_time.tv_sec) * 1000000 +
+		(end_time.tv_usec - start_time.tv_usec);
+	printk(KERN_ERR "usec_elapsed for mdelay(37) [%ld]\n", usec_elapsed);
+	printk(KERN_ERR "start time: [%ld]s [%ld]us \n", start_time.tv_sec, start_time.tv_usec);
+	printk(KERN_ERR "end   time: [%ld]s [%ld]us \n", end_time.tv_sec, end_time.tv_usec);
+
 	PDPRINTK("start[%ld] end[%ld] \n", start_count, end_count);
 	PDPRINTK("PLL input clock[%ld]Hz\n", pll_clock);