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Date: Mon, 08 Dec 2014 16:54:11 +0100
From: Paul Bolle <pebolle@...cali.nl>
To: Richard Leitner <dev@...l1n.net>
Cc: Arnd Bergmann <arnd@...db.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Prabhakar Lad <prabhakar.csengg@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] misc: ioc4: simplify wave period measurement in
clock_calibrate
On Mon, 2014-12-08 at 16:28 +0100, Richard Leitner wrote:
> The loop for measuring the square wave periods over some cycles is
> refactored to be more easily readable. This includes avoiding a
> "by-hand-implemented" for loop with a "real" one and adding some
> comments.
>
> Furthermore the following compiler warning is avoided by this patch:
> drivers/misc/ioc4.c: In function ‘ioc4_probe’:
> drivers/misc/ioc4.c:194:16: warning: ‘start’ may be used uninitialized
> in this function [-Wmaybe-uninitialized]
> period = (end - start) /
> ^
> drivers/misc/ioc4.c:148:11: note: ‘start’ was declared here
> uint64_t start, end, period;
> ^
>
> Signed-off-by: Richard Leitner <dev@...l1n.net>
> ---
> A simplification of this loop was suggested by Andrew Morton [1].
> This is my first proposal of such a simplification.
>
> Furthermore I'm not sure if the commit message is sufficient.
> Please give me also some feedback on it.
>
> If this simplification is not needed only initializing start to
> ktime_get_ns() would fix the compiler warning too.
>
> [1] https://lkml.org/lkml/2014/12/5/76
> ---
> drivers/misc/ioc4.c | 31 ++++++++++++++++---------------
> 1 file changed, 16 insertions(+), 15 deletions(-)
>
> diff --git a/drivers/misc/ioc4.c b/drivers/misc/ioc4.c
> index 3336ddc..8758d03 100644
> --- a/drivers/misc/ioc4.c
> +++ b/drivers/misc/ioc4.c
> @@ -144,9 +144,9 @@ ioc4_clock_calibrate(struct ioc4_driver_data *idd)
> {
> union ioc4_int_out int_out;
> union ioc4_gpcr gpcr;
> - unsigned int state, last_state = 1;
> + unsigned int state, last_state;
> uint64_t start, end, period;
> - unsigned int count = 0;
> + unsigned int count;
>
> /* Enable output */
> gpcr.raw = 0;
> @@ -167,19 +167,20 @@ ioc4_clock_calibrate(struct ioc4_driver_data *idd)
> mmiowb();
>
> /* Check square wave period averaged over some number of cycles */
> - do {
> - int_out.raw = readl(&idd->idd_misc_regs->int_out.raw);
> - state = int_out.fields.int_out;
> - if (!last_state && state) {
> - count++;
> - if (count == IOC4_CALIBRATE_END) {
> - end = ktime_get_ns();
> - break;
> - } else if (count == IOC4_CALIBRATE_DISCARD)
> - start = ktime_get_ns();
> - }
> - last_state = state;
> - } while (1);
> + start = ktime_get_ns();
> + state = 1; /* make sure the first read isn't a rising edge */
> + for (count = 0; count <= IOC4_CALIBRATE_END; count++) {
> + do { /* wait for a rising edge */
> + last_state = state;
> + int_out.raw = readl(&idd->idd_misc_regs->int_out.raw);
> + state = int_out.fields.int_out;
> + } while (last_state || !state);
> +
> + /* discard the first few cycles */
> + if (count == IOC4_CALIBRATE_DISCARD)
> + start = ktime_get_ns();
> + }
> + end = ktime_get_ns();
>
> /* Calculation rearranged to preserve intermediate precision.
> * Logically:
I've had the patch pasted at the end of this message in my local stack
of patches for some time now. It uses another approach: by using a
helper function things become much simpler. I _think_ it doesn't
introduce any functional changes but still need to a quiet day to make
sure that is correct.
Hope this helps,
Paul Bolle
---
>From 6fd7078b622323a2d9b72c9fffad347a1321204d Mon Sep 17 00:00:00 2001
From: Paul Bolle <pebolle@...cali.nl>
Date: Tue, 19 Aug 2014 09:11:40 +0200
Subject: [PATCH] ioc4: silence GCC warning
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Building ioc4.o triggers a GCC warning since v3.17-rc1:
drivers/misc/ioc4.c: In function ‘ioc4_probe’:
drivers/misc/ioc4.c:194:16: warning: ‘start’ may be used uninitialized in this function [-Wmaybe-uninitialized]
period = (end - start) /
^
drivers/misc/ioc4.c:148:11: note: ‘start’ was declared here
uint64_t start, end, period;
^
This is a false positive. Apparently the recent change to use
ktime_get_ns() makes it harder for GCC to analyze the codeflow.
Adding a small (inline) function that only returns after a certain
number of wave cycles have been seen allows to reorder the code a bit.
And after reordering it is clearer for both the compiler and the human
reader what's going on here. So let's do that.
Not-yet-signed-off-by: Paul Bolle <pebolle@...cali.nl>
---
drivers/misc/ioc4.c | 39 +++++++++++++++++++++------------------
1 file changed, 21 insertions(+), 18 deletions(-)
diff --git a/drivers/misc/ioc4.c b/drivers/misc/ioc4.c
index 3336ddca45ac..e8209fa78713 100644
--- a/drivers/misc/ioc4.c
+++ b/drivers/misc/ioc4.c
@@ -122,11 +122,25 @@ ioc4_unregister_submodule(struct ioc4_submodule *is)
#define IOC4_CALIBRATE_DEFAULT \
(1000*IOC4_EXTINT_COUNT_DIVISOR/IOC4_CALIBRATE_DEFAULT_MHZ)
-#define IOC4_CALIBRATE_END \
- (IOC4_CALIBRATE_CYCLES + IOC4_CALIBRATE_DISCARD)
-
#define IOC4_INT_OUT_MODE_TOGGLE 0x7 /* Toggle INT_OUT every COUNT+1 ticks */
+/* return after count wave cycles have been seen */
+static inline void
+ioc4_clock_wave_cycles(struct ioc4_driver_data *idd, unsigned int count)
+{
+ union ioc4_int_out int_out;
+ unsigned int state, last_state = 1;
+ unsigned int i = 0;
+
+ do {
+ int_out.raw = readl(&idd->idd_misc_regs->int_out.raw);
+ state = int_out.fields.int_out;
+ if (!last_state && state)
+ i++;
+ last_state = state;
+ } while (i < count);
+}
+
/* Determines external interrupt output clock period of the PCI bus an
* IOC4 is attached to. This value can be used to determine the PCI
* bus speed.
@@ -144,9 +158,7 @@ ioc4_clock_calibrate(struct ioc4_driver_data *idd)
{
union ioc4_int_out int_out;
union ioc4_gpcr gpcr;
- unsigned int state, last_state = 1;
uint64_t start, end, period;
- unsigned int count = 0;
/* Enable output */
gpcr.raw = 0;
@@ -167,19 +179,10 @@ ioc4_clock_calibrate(struct ioc4_driver_data *idd)
mmiowb();
/* Check square wave period averaged over some number of cycles */
- do {
- int_out.raw = readl(&idd->idd_misc_regs->int_out.raw);
- state = int_out.fields.int_out;
- if (!last_state && state) {
- count++;
- if (count == IOC4_CALIBRATE_END) {
- end = ktime_get_ns();
- break;
- } else if (count == IOC4_CALIBRATE_DISCARD)
- start = ktime_get_ns();
- }
- last_state = state;
- } while (1);
+ ioc4_clock_wave_cycles(idd, IOC4_CALIBRATE_DISCARD);
+ start = ktime_get_ns();
+ ioc4_clock_wave_cycles(idd, IOC4_CALIBRATE_COUNT);
+ end = ktime_get_ns();
/* Calculation rearranged to preserve intermediate precision.
* Logically:
--
1.9.3
--
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