linux-kernel - Re: [PATCH 3/3] hvc_dcc: Simplify assembly for v6 and v7 ARM

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Message-id: <alpine.LFD.2.00.1012201644110.10437@xanadu.home>
Date:	Mon, 20 Dec 2010 16:44:56 -0500 (EST)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Greg Kroah-Hartman <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Tony Lindgren <tony@...mide.com>,
	Arnd Bergmann <arnd@...db.de>,
	Daniel Walker <dwalker@...eaurora.org>
Subject: Re: [PATCH 3/3] hvc_dcc: Simplify assembly for v6 and v7 ARM

On Mon, 20 Dec 2010, Stephen Boyd wrote:

> The inline assembly differences for v6 vs. v7 in the hvc_dcc
> driver are purely optimizations. On a v7 processor, an mrc with
> the pc sets the condition codes to the 28-31 bits of the register
> being read. It just so happens that the TX/RX full bits the DCC
> driver is testing for are high enough in the register to be put
> into the condition codes. On a v6 processor, this "feature" isn't
> implemented and thus we have to do the usual read, mask, test
> operations to check for TX/RX full.
> 
> Since we already test the RX/TX full bits before calling
> __dcc_getchar() and __dcc_putchar() we don't actually need to do
> anything special for v7 over v6. The only difference is in
> hvc_dcc_get_chars(). We would test RX full, poll RX full, and
> then read a character from the buffer, whereas now we will test
> RX full, read a character from the buffer, and then test RX full
> again for the second iteration of the loop. It doesn't seem
> possible for the buffer to go from full to empty between testing
> the RX full and reading a character. Therefore, replace the v7
> versions with the v6 versions and everything works the same.
> 
> Cc: Tony Lindgren <tony@...mide.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Nicolas Pitre <nicolas.pitre@...aro.org>
> Cc: Daniel Walker <dwalker@...eaurora.org>
> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>

Acked-by: Nicolas Pitre <nicolas.pitre@...aro.org>

> ---
>  drivers/char/hvc_dcc.c |   24 ------------------------
>  1 files changed, 0 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/char/hvc_dcc.c b/drivers/char/hvc_dcc.c
> index ad23cc8..435f6fa 100644
> --- a/drivers/char/hvc_dcc.c
> +++ b/drivers/char/hvc_dcc.c
> @@ -40,19 +40,6 @@ static inline u32 __dcc_getstatus(void)
>  }
>  
>  
> -#if defined(CONFIG_CPU_V7)
> -static inline char __dcc_getchar(void)
> -{
> -	char __c;
> -
> -	asm volatile("get_wait:	mrc p14, 0, pc, c0, c1, 0                  \n\
> -			bne get_wait                                       \n\
> -			mrc p14, 0, %0, c0, c5, 0	@ read comms data reg"
> -		: "=r" (__c) : : "cc");
> -
> -	return __c;
> -}
> -#else
>  static inline char __dcc_getchar(void)
>  {
>  	char __c;
> @@ -62,24 +49,13 @@ static inline char __dcc_getchar(void)
>  
>  	return __c;
>  }
> -#endif
>  
> -#if defined(CONFIG_CPU_V7)
> -static inline void __dcc_putchar(char c)
> -{
> -	asm volatile("put_wait:	mrc p14, 0, pc, c0, c1, 0         \n\
> -			bcs put_wait                              \n\
> -			mcr p14, 0, %0, c0, c5, 0                   "
> -	: : "r" (c) : "cc");
> -}
> -#else
>  static inline void __dcc_putchar(char c)
>  {
>  	asm volatile("mcr p14, 0, %0, c0, c5, 0	@ write a char"
>  		: /* no output register */
>  		: "r" (c));
>  }
> -#endif
>  
>  static int hvc_dcc_put_chars(uint32_t vt, const char *buf, int count)
>  {
> -- 
> 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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