Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752373Ab0LAFaN (ORCPT ); Wed, 1 Dec 2010 00:30:13 -0500 Received: from wolverine01.qualcomm.com ([199.106.114.254]:9469 "EHLO wolverine01.qualcomm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750735Ab0LAFaL (ORCPT ); Wed, 1 Dec 2010 00:30:11 -0500 X-IronPort-AV: E=McAfee;i="5400,1158,6183"; a="64983005" Message-ID: <4CF5DD63.40803@codeaurora.org> Date: Tue, 30 Nov 2010 21:30:11 -0800 From: Stephen Boyd User-Agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv:1.9.2.12) Gecko/20101027 Thunderbird/3.1.6 MIME-Version: 1.0 To: Daniel Walker CC: linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Tony Lindgren , Arnd Bergmann , Nicolas Pitre , Greg Kroah-Hartman , Mike Frysinger , Andrew Morton , Alan Cox , Randy Dunlap , FUJITA Tomonori , linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH] drivers: char: hvc: add arm JTAG DCC console support References: <1291145141-18301-1-git-send-email-dwalker@codeaurora.org> In-Reply-To: <1291145141-18301-1-git-send-email-dwalker@codeaurora.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6769 Lines: 185 On 11/30/2010 11:25 AM, Daniel Walker wrote: > @@ -682,6 +682,15 @@ config HVC_UDBG > select HVC_DRIVER > default n > > +config HVC_DCC > + bool "ARM JTAG DCC console" > + depends on ARM > + select HVC_DRIVER > + help > + This console uses the JTAG DCC on ARM to create a console under the HVC Looks like you added one too many spaces for indent here. > diff --git a/drivers/char/hvc_dcc.c b/drivers/char/hvc_dcc.c > new file mode 100644 > index 0000000..6470f63 > --- /dev/null > +++ b/drivers/char/hvc_dcc.c > +static inline u32 __dcc_getstatus(void) > +{ > + u32 __ret; > + > + asm("mrc p14, 0, %0, c0, c1, 0 @ read comms ctrl reg" > + : "=r" (__ret) : : "cc"); > + > + return __ret; > +} Without marking this asm volatile my compiler decides it can cache the value of __ret in a register and then check the value of it continually in hvc_dcc_put_chars() (I had to replace get_wait/put_wait with 1 and fixup the branch otherwise my disassembler barfed on __dcc_(get|put)char). 00000000 : 0: ee103e11 mrc 14, 0, r3, cr0, cr1, {0} 4: e3a0c000 mov ip, #0 ; 0x0 8: e2033202 and r3, r3, #536870912 ; 0x20000000 c: ea000006 b 2c 10: e3530000 cmp r3, #0 ; 0x0 14: 1afffffd bne 10 18: e7d1000c ldrb r0, [r1, ip] 1c: ee10fe11 mrc 14, 0, pc, cr0, cr1, {0} 20: 2afffffd bcs 1c 24: ee000e15 mcr 14, 0, r0, cr0, cr5, {0} 28: e28cc001 add ip, ip, #1 ; 0x1 2c: e15c0002 cmp ip, r2 30: bafffff6 blt 10 34: e1a00002 mov r0, r2 38: e12fff1e bx lr As you can see, the value of the mrc is checked against DCC_STATUS_TX (bit 29) and then stored in r3 for later use. Marking this volatile produces the following: 00000000 : 0: e3a03000 mov r3, #0 ; 0x0 4: ea000007 b 28 8: ee100e11 mrc 14, 0, r0, cr0, cr1, {0} c: e3100202 tst r0, #536870912 ; 0x20000000 10: 1afffffc bne 8 14: e7d10003 ldrb r0, [r1, r3] 18: ee10fe11 mrc 14, 0, pc, cr0, cr1, {0} 1c: 2afffffd bcs 18 20: ee000e15 mcr 14, 0, r0, cr0, cr5, {0} 24: e2833001 add r3, r3, #1 ; 0x1 28: e1530002 cmp r3, r2 2c: bafffff5 blt 8 30: e1a00002 mov r0, r2 34: e12fff1e bx lr which looks better. I marked all the asm in this driver as volatile. Is that correct? > +#if defined(CONFIG_CPU_V7) > +static inline char __dcc_getchar(void) > +{ > + char __c; > + > + asm("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; > + > + asm("mrc p14, 0, %0, c0, c5, 0 @ read comms data reg" > + : "=r" (__c)); > + > + return __c; > +} > +#endif > + > +#if defined(CONFIG_CPU_V7) > +static inline void __dcc_putchar(char c) > +{ > + asm("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("mcr p14, 0, %0, c0, c5, 0 @ write a char" > + : /* no output register */ > + : "r" (c)); > +} > +#endif > + I don't think both the v7 and v6 functions are necessary. It seems I can get away with just the second version of __dcc_(get|put)char() on a v7. The mrc p14, 0, pc, c0, c1, 0 will assign the top 4 bits (31-28) to the condition codes NZCV on v7. It also looks like on an ARM11 (a v6) will also do the same thing if I read the manuals right. The test in the inline assembly is saying, wait for a character to be ready or wait for a character to be read then actually write a character or read one. The code in hvc_dcc_put_chars() is already doing the same thing, albeit in a slightly different form. Instead of getting the status bits put into the condition codes and looping with bne or bcs it will read the register, and it with bit 29 or bit 28 to see if it should wait and then continue with the writing/reading. I think you can just drop the looping for the v7 version of the functions and have this driver work on v6 and v7. Alternatively, you can make some function that says tx buffer is empty, rx buffer is full or something but I don't see how saving a couple instructions buys us much when we can have one driver for v6 and v7. I see that Tony Lindgren modified the DCC macros for v7 in commit 200b7a8 (ARM: 5884/1: arm: Fix DCC console for v7, 2010-01-19). I'm not sure why though, since it seems that a v6 and a v7 should really do the same thing by waiting for the buffers to be ready before filling them or reading them. Which probably means we can get low-level dcc debugging on all targets if I'm not mistaken. > +static int hvc_dcc_put_chars(uint32_t vt, const char *buf, int count) > +{ > + int i; > + > + for (i = 0; i < count; i++) { > + while (__dcc_getstatus() & DCC_STATUS_TX) > + cpu_relax(); > + > + __dcc_putchar((char)(buf[i] & 0xFF)); Is this & 0xFF and cast to char unnecessary? buf is a char array, and chars are always 8 bits. Can't we just do __dcc_putchar(buf[i])? > +static int hvc_dcc_get_chars(uint32_t vt, char *buf, int count) > +{ > + int i; > + > + for (i = 0; i < count; ++i) { > + int c = -1; > + > + if (__dcc_getstatus() & DCC_STATUS_RX) > + c = __dcc_getchar(); > + if (c < 0) > + break; > + buf[i] = c; > + } I think this for loop can be simplified. __dcc_getchar() returns a char. It never returns -1, so the check for c < 0 can't be taken if __dcc_getstatus() & DCC_STATUS_RX is true. The only case you break the loop in then is if __dcc_getstatus() & DCC_STATUS_RX is false. So you can have a simple if-else and assign buf[i] in the if branch and break in the else branch. -- Sent by an employee of the Qualcomm Innovation Center, Inc. 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