From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from cpsmtpb-ews10.kpnxchange.com ([213.75.39.15]) by merlin.infradead.org with esmtp (Exim 4.76 #1 (Red Hat Linux)) id 1SwA27-0006UK-H0 for barebox@lists.infradead.org; Tue, 31 Jul 2012 10:50:36 +0000 Message-ID: <5017B839.7070806@erwinrol.com> Date: Tue, 31 Jul 2012 12:49:29 +0200 From: Erwin Rol MIME-Version: 1.0 References: <50170C6A.2000101@erwinrol.com> In-Reply-To: <50170C6A.2000101@erwinrol.com> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: barebox-bounces@lists.infradead.org Errors-To: barebox-bounces+u.kleine-koenig=pengutronix.de@lists.infradead.org Subject: Re: x86 weirdness To: barebox@lists.infradead.org Hey x86-asm-experts, On 31-7-2012 0:36, Erwin Rol wrote: > Hallo, > > after I had successfully "ported" barebox to about 5 different embedded > x86 (mostly N270 and Z510 Atom boards) I ran into problems with a > Celeron SU2300 CPU based board. After a lot of trail and error, which > included the implementation of a BIOS based console cause the board does > not have a RS232 port, it looked like the global variables were not what > I expected them to be, namely zero. > > So in arch/x86/boot/main_entry.c i changed to following; > > /** > * Called plainly from assembler that switches from real to flat mode > * > * @note The C environment isn't initialized yet > */ > void uboot_entry(void) > { > #if 0 > /* clear the BSS first */ > memset(__bss_start, 0x00, __bss_stop - __bss_start); > #else > unsigned char* ptr; > for (ptr = __bss_start; ptr < __bss_stop; ptr++) > *ptr = 0x00; > #endif > > > start_barebox(); > } > > > After doing that it works. I have not yet looked into the resulting asm > to find the differences, but could someone have an explanation for why > the memset doesn't work and the simple loop does work? So when I use the memset code the asm looks like this below. As you can see the memset gets its 3 parameters from the stack (see comments below in the asm src). But when you look at the location when memset is called it looks like the caller tries to pass the parameters in registers. After looking into the Makefile I noticed the arch/x86/boot/* sources get compiled with -mregparm=3, but the other sources use normal calling convention. This means code from arch/x86/boot/* can not call any other barebox functions when they are not of type void f(void). The file arch/x86/boot/regs.c also calls memset but it seems the compiler notices that regs.c is 16bit code and so it inlines a 16bit memset version (I at least don't see a call function). Is there any GCC or LD magic we can use to prevent error like this? Wouldn't it be better to _not_ change the calling convention in the Makefile, but mark the functions that need it with __attribute__ ? 00023e98 : 23e98: 8e d9 mov %ecx,%ds 23e9a: 8e c1 mov %ecx,%es 23e9c: 8e e1 mov %ecx,%fs 23e9e: 8e e9 mov %ecx,%gs 23ea0: 8e d1 mov %ecx,%ss 23ea2: bc c8 f8 03 00 mov $0x3f8c8,%esp 23ea7: 0f 00 df ltr %di 23eaa: 31 c9 xor %ecx,%ecx 23eac: 31 d2 xor %edx,%edx 23eae: 31 db xor %ebx,%ebx 23eb0: 31 ed xor %ebp,%ebp 23eb2: 31 ff xor %edi,%edi 23eb4: 0f 00 d1 lldt %cx 23eb7: e9 00 00 00 00 jmp 23ebc 00023ebc : // calculate (3rd parameter) size and put it in %ecx 23ebc: b9 c8 78 03 00 mov $0x378c8,%ecx 23ec1: 81 e9 70 a2 02 00 sub $0x2a270,%ecx // clear %edx (2nd paremeter) 23ec7: 31 d2 xor %edx,%edx // start address in %eax (1st parameter) 23ec9: b8 70 a2 02 00 mov $0x2a270,%eax 23ece: e8 e6 0b ff ff call 14ab9 23ed3: e9 4c a4 fe ff jmp e324 00014ab9 : 14ab9: 55 push %ebp 14aba: 89 e5 mov %esp,%ebp 14abc: 53 push %ebx // get parameter 1 (dst address) 14abd: 8b 45 08 mov 0x8(%ebp),%eax // get parameter 2 (src byte to fill) 14ac0: 8b 5d 0c mov 0xc(%ebp),%ebx // get parameter 3 (size) 14ac3: 8b 4d 10 mov 0x10(%ebp),%ecx 14ac6: 31 d2 xor %edx,%edx 14ac8: eb 04 jmp 14ace 14aca: 88 1c 10 mov %bl,(%eax,%edx,1) 14acd: 42 inc %edx 14ace: 39 ca cmp %ecx,%edx 14ad0: 75 f8 jne 14aca 14ad2: 5b pop %ebx 14ad3: 5d pop %ebp 14ad4: c3 ret _______________________________________________ barebox mailing list barebox@lists.infradead.org http://lists.infradead.org/mailman/listinfo/barebox