Re: [PATCH v5 1/4] usb: dbc: early driver for xhci debug capability

[Lu Baolu <baolu.lu@linux.intel.com>]

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Hi Ingo,

On 01/22/2017 05:04 PM, Ingo Molnar wrote:
> * Lu Baolu <baolu.lu@linux.intel.com> wrote:
>
>>>> +static void xdbc_runtime_delay(unsigned long count)
>>>> +{
>>>> +	udelay(count);
>>>> +}
>>>> +static void (*xdbc_delay)(unsigned long) = xdbc_early_delay;
>>> Is this udelay() complication really necessary? udelay() should work fine even in 
>>> early code. It might not be precisely calibrated, but should be good enough.
>> I tried udelay() in the early code. It's not precise enough for the
>> hardware handshaking.
> Possibly because on x86 early udelay() did not work at all - i.e. there's no delay 
> whatsoever.

Yes.

>
> Could you try it on top of this commit in tip:timers/core:
>
>   4c45c5167c95 x86/timer: Make delay() work during early bootup
>
> ?

I tried tip:timers/core. It's not precise enough for my context either.

__const_udelay().

157 inline void __const_udelay(unsigned long xloops)
158 {
159         unsigned long lpj = this_cpu_read(cpu_info.loops_per_jiffy) ? : loops_per_jiffy;
160         int d0;
161
162         xloops *= 4;
163         asm("mull %%edx"
164                 :"=d" (xloops), "=&a" (d0)
165                 :"1" (xloops), "0" (lpj * (HZ / 4)));
166
167         __delay(++xloops);
168 }


In my early  code, loops_per_jiffy is not initialized yet. Hence "lpj" for the asm line
is 4096 (default value).

The  cpu_info.loops_per_jiffy actually reads 8832000 after initialization. They are
about 2000 times different.

I did a hacky test in kernel to check the difference between these two different
"lpj" values. (The hacky patch is attached.) Below is the output for 100ms delay.

[    2.494751] udelay_test uninitialized ---->start
[    2.494820] udelay_test uninitialized ---->end
[    2.494828] udelay_test initialized ---->start
[    2.595234] udelay_test initialized ---->end

For 100ms delay, udelay() with uninitialized loops_per_jiffy only gives a delay of
only 69us.

Best regards,
Lu Baolu

[-- Attachment #2: kernel-hack.patch --]
[-- Type: text/x-patch, Size: 2325 bytes --]

diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
index a8e91ae..ffc2874 100644
--- a/arch/x86/lib/delay.c
+++ b/arch/x86/lib/delay.c
@@ -168,6 +168,36 @@ inline void __const_udelay(unsigned long xloops)
 }
 EXPORT_SYMBOL(__const_udelay);
 
+void udelay_uninitialized(unsigned long xloops)
+{
+	unsigned long lpj = (1<<12);
+	int d0;
+
+	xloops *= 0x10c7ul;
+	xloops *= 4;
+	asm("mull %%edx"
+		:"=d" (xloops), "=&a" (d0)
+		:"1" (xloops), "0" (lpj * (HZ / 4)));
+
+	delay_loop(++xloops);
+}
+EXPORT_SYMBOL(udelay_uninitialized);
+
+void udelay_initialized(unsigned long xloops)
+{
+	unsigned long lpj = this_cpu_read(cpu_info.loops_per_jiffy);
+	int d0;
+
+	xloops *= 0x10c7ul;
+	xloops *= 4;
+	asm("mull %%edx"
+		:"=d" (xloops), "=&a" (d0)
+		:"1" (xloops), "0" (lpj * (HZ / 4)));
+
+	delay_loop(++xloops);
+}
+EXPORT_SYMBOL(udelay_initialized);
+
 void __udelay(unsigned long usecs)
 {
 	__const_udelay(usecs * 0x000010c7); /* 2**32 / 1000000 (rounded up) */
diff --git a/drivers/usb/host/xhci-pci.c b/drivers/usb/host/xhci-pci.c
index 954abfd..b6a7437 100644
--- a/drivers/usb/host/xhci-pci.c
+++ b/drivers/usb/host/xhci-pci.c
@@ -302,6 +302,21 @@ static int xhci_pci_probe(struct pci_dev *dev, const struct pci_device_id *id)
 	/* USB-2 and USB-3 roothubs initialized, allow runtime pm suspend */
 	pm_runtime_put_noidle(&dev->dev);
 
+	do {
+		int count = 1000;
+
+		pr_notice("udelay_test uninitialized ---->start\n");
+		while (count-- > 0)
+			udelay_uninitialized(100);
+		pr_notice("udelay_test uninitialized ---->end\n");
+
+		count = 1000;
+		pr_notice("udelay_test initialized ---->start\n");
+		while (count-- > 0)
+			udelay_initialized(100);
+		pr_notice("udelay_test initialized ---->end\n");
+	} while (0);
+
 	return 0;
 
 put_usb3_hcd:
diff --git a/include/asm-generic/delay.h b/include/asm-generic/delay.h
index 0f79054..200ab55 100644
--- a/include/asm-generic/delay.h
+++ b/include/asm-generic/delay.h
@@ -9,6 +9,8 @@ extern void __udelay(unsigned long usecs);
 extern void __ndelay(unsigned long nsecs);
 extern void __const_udelay(unsigned long xloops);
 extern void __delay(unsigned long loops);
+extern void udelay_uninitialized(unsigned long xloops);
+extern void udelay_initialized(unsigned long xloops);
 
 /*
  * The weird n/20000 thing suppresses a "comparison is always false due to