Re: [PATCH 1/3] [ARM] Translate delay.S into (mostly) C

[Daniel Walker]

On Mon, 2010-09-27 at 20:33 -0700, Stephen Boyd wrote:
> In the next patch we're going to allow machines to override the
> __delay() implementation at runtime so they can implement a timer
> based __delay() routine. It's easier to do this using C, so lets
> write udelay and friends in C.

You shouldn't really reference this series in this comment. You have to
look at this as a changeset comment. So you really want to describe what
this change is doing not what the over all series is doing.

It would be better to say something like,

"We're implementing this in C to give us further flexibility in allowing
overrides."

But don't references "next patch" or "this series" , but you can do that
in the intro email.

> We lose the #if 0 code, which according to Russell is used "to
> make the delay loop more stable and predictable on older CPUs"
> (see http://article.gmane.org/gmane.linux.kernel/888867 for more
> info). We shouldn't be too worried though, since the next patch
> adds functionality to allow a machine to set the __delay() loop
> themselves, therefore allowing machines to resurrect the
> commented out code if they need it.
> 
> bloat-o-meter shows an increase of 12 bytes. Further inspection
> of the assembly shows GCC copying the loops_per_jiffy pointer and
> the magic HZ value to the ends of __const_udelay() and _delay()
> thus contributing an extra 4 and 8 bytes of data to each
> function. These two values weren't taken into account in the
> delay.S version since they weren't part of the function in nm's
> eyes. This means we only really gained an extra 4 bytes due to
> GCC's decision to duplicate the loops_per_jiffy pointer in
> __const_udelay.
> 
>  $ scripts/bloat-o-meter vmlinux.orig vmlinux.new
>  add/remove: 0/0 grow/shrink: 2/0 up/down: 12/0 (12)
>  function                                     old     new   delta
>  __udelay                                      48      56      +8
>  __const_udelay                                40      44      +4

I think the "size" command might be better for this type of stuff. It
should give you the same output (or similar).. It's just used more
frequently. 

Is this an -Os kernel, or -O2 ?

> Signed-off-by: Stephen Boyd <sboyd@xxxxxxxxxxxxxx>
> Reviewed-by: Saravana Kannan <skannan@xxxxxxxxxxxxxx>
> ---
> 
> So maybe we can make the magic HZ value into a #define? UDELAY_SCALAR?
> UDELAY_MAGIC?
> 
>  arch/arm/include/asm/delay.h |    2 +-
>  arch/arm/kernel/armksyms.c   |    4 --
>  arch/arm/lib/delay.S         |   65 ------------------------------------------
>  arch/arm/lib/delay.c         |   57 ++++++++++++++++++++++++++++++++++++
>  4 files changed, 58 insertions(+), 70 deletions(-)
>  delete mode 100644 arch/arm/lib/delay.S
>  create mode 100644 arch/arm/lib/delay.c
> 
> diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
> index b2deda1..ccc5ed5 100644
> --- a/arch/arm/include/asm/delay.h
> +++ b/arch/arm/include/asm/delay.h
> @@ -8,7 +8,7 @@
>  
>  #include <asm/param.h>	/* HZ */
>  
> -extern void __delay(int loops);
> +extern void __delay(unsigned long loops);
>  
>  /*
>   * This function intentionally does not exist; if you see references to
> diff --git a/arch/arm/kernel/armksyms.c b/arch/arm/kernel/armksyms.c
> index 8214bfe..259e549 100644
> --- a/arch/arm/kernel/armksyms.c
> +++ b/arch/arm/kernel/armksyms.c
> @@ -52,10 +52,6 @@ extern void fpundefinstr(void);
>  
>  EXPORT_SYMBOL(__backtrace);
>  
> -	/* platform dependent support */
> -EXPORT_SYMBOL(__udelay);
> -EXPORT_SYMBOL(__const_udelay);
> -
>  	/* networking */
>  EXPORT_SYMBOL(csum_partial);
>  EXPORT_SYMBOL(csum_partial_copy_from_user);
> diff --git a/arch/arm/lib/delay.S b/arch/arm/lib/delay.S
> deleted file mode 100644
> index 8d6a876..0000000
> --- a/arch/arm/lib/delay.S
> +++ /dev/null
> @@ -1,65 +0,0 @@
> -/*
> - *  linux/arch/arm/lib/delay.S
> - *
> - *  Copyright (C) 1995, 1996 Russell King
> - *
> - * This program is free software; you can redistribute it and/or modify
> - * it under the terms of the GNU General Public License version 2 as
> - * published by the Free Software Foundation.
> - */
> -#include <linux/linkage.h>
> -#include <asm/assembler.h>
> -#include <asm/param.h>
> -		.text
> -
> -.LC0:		.word	loops_per_jiffy
> -.LC1:		.word	(2199023*HZ)>>11
> -
> -/*
> - * r0  <= 2000
> - * lpj <= 0x01ffffff (max. 3355 bogomips)
> - * HZ  <= 1000
> - */
> -
> -ENTRY(__udelay)
> -		ldr	r2, .LC1
> -		mul	r0, r2, r0
> -ENTRY(__const_udelay)				@ 0 <= r0 <= 0x7fffff06
> -		ldr	r2, .LC0
> -		ldr	r2, [r2]		@ max = 0x01ffffff
> -		mov	r0, r0, lsr #14		@ max = 0x0001ffff
> -		mov	r2, r2, lsr #10		@ max = 0x00007fff
> -		mul	r0, r2, r0		@ max = 2^32-1
> -		movs	r0, r0, lsr #6
> -		moveq	pc, lr
> -
> -/*
> - * loops = r0 * HZ * loops_per_jiffy / 1000000
> - *
> - * Oh, if only we had a cycle counter...
> - */
> -
> -@ Delay routine
> -ENTRY(__delay)
> -		subs	r0, r0, #1
> -#if 0
> -		movls	pc, lr
> -		subs	r0, r0, #1
> -		movls	pc, lr
> -		subs	r0, r0, #1
> -		movls	pc, lr
> -		subs	r0, r0, #1
> -		movls	pc, lr
> -		subs	r0, r0, #1
> -		movls	pc, lr
> -		subs	r0, r0, #1
> -		movls	pc, lr
> -		subs	r0, r0, #1
> -		movls	pc, lr
> -		subs	r0, r0, #1
> -#endif
> -		bhi	__delay
> -		mov	pc, lr
> -ENDPROC(__udelay)
> -ENDPROC(__const_udelay)
> -ENDPROC(__delay)
> diff --git a/arch/arm/lib/delay.c b/arch/arm/lib/delay.c
> new file mode 100644
> index 0000000..5ee0adc
> --- /dev/null
> +++ b/arch/arm/lib/delay.c
> @@ -0,0 +1,57 @@
> +/*
> + *  Originally from linux/arch/arm/lib/delay.S
> + *
> + *  Copyright (C) 1995, 1996 Russell King
> + *  Copyright (c) 2010, Code Aurora Forum. All rights reserved.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + */
> +#include <linux/module.h>
> +#include <linux/delay.h>
> +
> +/*
> + * loops = usecs * HZ * loops_per_jiffy / 1000000
> + *
> + * Oh, if only we had a cycle counter...
> + */
> +void __delay(unsigned long loops)
> +{
> +	asm volatile(
> +	"1:	subs %0, %0, #1 \n"
> +	"	bhi 1b		\n"
> +	: /* No output */
> +	: "r" (loops)
> +	);
> +}
> +EXPORT_SYMBOL(__delay);
> +
> +/*
> + * 0 <= xloops <= 0x7fffff06
> + * loops_per_jiffy <= 0x01ffffff (max. 3355 bogomips)

As long as your doing a re-write may as well add some real text here,
and for the others.

> + */
> +void __const_udelay(unsigned long xloops)
> +{
> +	unsigned long lpj;
> +	unsigned long loops;
> +
> +	xloops >>= 14;			/* max = 0x01ffffff */
> +	lpj = loops_per_jiffy >> 10;	/* max = 0x0001ffff */
> +	loops = lpj * xloops;		/* max = 0x00007fff */
> +	loops >>= 6;			/* max = 2^32-1 */
> +
> +	if (loops)
> +		__delay(loops);

likely/unlikely ?

> +}
> +EXPORT_SYMBOL(__const_udelay);
> +
> +/*
> + * usecs  <= 2000
> + * HZ  <= 1000
> + */
> +void __udelay(unsigned long usecs)
> +{
> +	__const_udelay(usecs * ((2199023*HZ)>>11));
> +}
> +EXPORT_SYMBOL(__udelay);

-- 

Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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