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

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Date:	Wed, 06 Oct 2010 13:24:37 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Daniel Walker <dwalker@...eaurora.org>
CC:	Nicolas Pitre <nico@...xnic.net>,
	Russell King <linux@....linux.org.uk>,
	Kevin Hilman <khilman@...prootsystems.com>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Saravana Kannan <skannan@...eaurora.org>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Colin Cross <ccross@...roid.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] [ARM] Translate delay.S into (mostly) C

 On 10/06/2010 12:35 PM, Daniel Walker wrote:
> Is it possible to do all this in assembly ? Can't you have the default
> implementation using this assembly with different function names, then
> just set the assembly function names in C code someplace?

Sure we could do that. I went this route because adding the timer based
delay code was a copy paste instead of a copy translate. Actually, after
adding the set_delay_fn code __const_udelay and __delay aren't inlined
into __udelay anymore so we're back to the noinline behavior except
we're missing interleaving.

Finally, I thought it would be clearer what was going on if it was in C
as opposed to assembly. How bad is a branch as opposed to fall through.
And more importantly, how bad is a push/pop?

00000000 <delay_loop>:
   0:   e2500001        subs    r0, r0, #1      ; 0x1
   4:   8afffffd        bhi     0 <delay_loop>
   8:   e12fff1e        bx      lr

0000000c <set_delay_fn>:
   c:   e59f3004        ldr     r3, [pc, #4]    ; 18 <set_delay_fn+0xc>
  10:   e5830000        str     r0, [r3]
  14:   e12fff1e        bx      lr
  18:   00000000        .word   0x00000000

0000001c <__delay>:
  1c:   e92d4010        push    {r4, lr}
  20:   e59f3008        ldr     r3, [pc, #8]    ; 30 <__delay+0x14>
  24:   e1a0e00f        mov     lr, pc
  28:   e593f000        ldr     pc, [r3]
  2c:   e8bd8010        pop     {r4, pc}
  30:   00000000        .word   0x00000000

00000034 <__const_udelay>:
  34:   e59f3018        ldr     r3, [pc, #24]   ; 54 <__const_udelay+0x20>
  38:   e1a00720        lsr     r0, r0, #14
  3c:   e5933000        ldr     r3, [r3]
  40:   e1a03523        lsr     r3, r3, #10
  44:   e0000093        mul     r0, r3, r0
  48:   e1b00320        lsrs    r0, r0, #6
  4c:   012fff1e        bxeq    lr
  50:   eafffffe        b       1c <__delay>
  54:   00000000        .word   0x00000000

00000058 <__udelay>:
  58:   e59f3004        ldr     r3, [pc, #4]    ; 64 <__udelay+0xc>
  5c:   e0000093        mul     r0, r3, r0
  60:   eafffffe        b       34 <__const_udelay>
  64:   0001a36e        .word   0x0001a36e


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

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