[PATCH] clocksource: dw_apb_timer_of: support timer-based delay

Dear Daniel,

On Mon, 2 Nov 2015 09:48:38 +0100
Daniel Lezcano  wrote:

On 11/02/2015 03:51 AM, Jisheng Zhang wrote:

quoted

Dear Daniel,

On Fri, 30 Oct 2015 13:37:01 +0100
Daniel Lezcano wrote:
 

quoted

On 10/30/2015 12:09 PM, Jisheng Zhang wrote:  

quoted

Dear Daniel,

On Fri, 30 Oct 2015 11:44:46 +0100
Daniel Lezcano <daniel.lezcano@....> wrote:
 

quoted

On 10/30/2015 09:27 AM, Jisheng Zhang wrote:  

quoted

Implement an ARM delay timer to be used for udelay(). This allows us to
skip the delay loop calibration at boot on Marvell BG2, BG2Q, BG2CD
platforms. And after this patch, udelay() will be unaffected by CPU
frequency changes.

Signed-off-by: Jisheng Zhang <redacted>
---
    drivers/clocksource/Kconfig           | 10 ++++++++++
    drivers/clocksource/dw_apb_timer_of.c | 16 ++++++++++++++++
    2 files changed, 26 insertions(+)

diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
index a7726db..7b081805 100644
--- a/drivers/clocksource/Kconfig
+++ b/drivers/clocksource/Kconfig

@@ -29,6 +29,16 @@ config DW_APB_TIMER_OF
    	select DW_APB_TIMER
    	select CLKSRC_OF

+config DW_APB_TIMER_BASED_DELAY
+	bool "DW APB timer based delay"
+	depends on ARM && DW_APB_TIMER_OF
+	default n
+	help
+	  This option enables support for using the DW APB timer to
+	  implement timer-based delay. It is useful for skiping the
+	  delay loop calibration at boot on some platforms. And the
+	  udelay() will be unaffected by CPU frequency changes.
+  

Why do you want it to be optional ?
 

Because in some platforms which has arm arch timer, this dw apb timer
delay isn't needed, the arch timer is better. So we want it be optional
so that the platforms which need this feature select it manually when config
the kernel.  

Correct me if I am wrong. If you have the arch timer, you don't need the  

Yes, I don't need the dw apb timer if we have arch timer,
 

quoted

dw apb timer at all, no ? So the selection would be arch arm timer *or*
dw_apb_timer ? not arch_arm_timer for delay and dw_apb_timer for
clockevents, right ?  

Yes, if we have arch timer, I prefer to use it for clockevent and delay.

Could you please provide suggestion how to handle this case?  

If I follow the logic of arch_arm_timer is better than dw_apb timer.

1. The arch_arm_timer is present

  => dw_apb timer is not used at all  

  CONFIG_ARM_ARCH_TIMER=y
  # CONFIG_DW_APB_TIMER is not set

2. The arch_arm_timer is *not* present

  => dw_apb_timer is used with delay code  

  # CONFIG_ARM_ARCH_TIMER is not set
  CONFIG_DW_APB_TIMER=y

In both cases, DW_APB_TIMER_BASED_DELAY is not needed.

The problem is register_current_timer_delay() is only available in ARM but
DW APB timer isn't only used in ARM platforms, but also ARM64 etc. where the
register_current_timer_delay() isn't defined. so I used DW_APB_TIMER_BASED_DELAY
to 

1. distinguish whether we need the dw apb timer based delay

2. distinguish whether we have register_current_timer_delay()

I have a solution which doesn't need DW_APB_TIMER_BASED_DELAY symbol -- put
all timer based delay code under CONFIG_ARM, I'm not sure whether this is
better. Could you please give suggestion?

Thanks in advance,
Jisheng