[U-Boot] [RFC PATCH v2] ARM: Avoid compiler optimization for usages of readb, writeb and friends.

Dirk Behme
2010-12-21T08:35:25Z

(Resend with corrected broken example)

On 21.12.2010 08:21, Albert ARIBAUD wrote:
> Hi Dirk,
>
> Le 21/12/2010 08:11, Dirk Behme a ?crit :
>
>> But the issue with drivers/mtd/nand/omap_gpmc.c (i.e. the additional
>> ldrb r3, [r3]) is still open? Has anybody tried to replace it with
>> a nop in the binary to be sure this is the root cause?
>
> Can you try and preprocess the C file for both the broken and working
> cases, then post the preprocessed C extract? Differences at the C level
> may help understanding differences at the asm level.

gcc version 4.5.1 (Sourcery G++ Lite 2010.09-50)

Work:
====

static void omap_nand_hwcontrol(struct mtd_info *mtd, int32_t cmd,
uint32_t ctrl)
{
register struct nand_chip *this = mtd->priv;
...
if (cmd != -1)

(*(volatile unsigned char *)(this->IO_ADDR_W) = (cmd));
}

if (cmd != NAND_CMD_NONE)
84: e3710001 cmn r1, #1
origwriteb(cmd, this->IO_ADDR_W);
88: 15933004 ldrne r3, [r3, #4]
8c: 120110ff andne r1, r1, #255 ; 0xff
90: 15c31000 strbne r1, [r3]
94: e12fff1e bx lr
...

Broken:
======

static void omap_nand_hwcontrol(struct mtd_info *mtd, int32_t cmd,
uint32_t ctrl)
{
register struct nand_chip *this = mtd->priv;

...

if (cmd != -1)
({ __asm__ __volatile__ ("" : : : "memory"); (*(volatile unsigned
char *)(this->IO_ADDR_W) = (cmd)); });
}

if (cmd != NAND_CMD_NONE)
84: e3710001 cmn r1, #1
88: 012fff1e bxeq lr
writeb(cmd, this->IO_ADDR_W);
8c: e5933004 ldr r3, [r3, #4]
90: e20110ff and r1, r1, #255 ; 0xff
94: e5c31000 strb r1, [r3]
98: e5d33000 ldrb r3, [r3]
9c: e12fff1e bx lr

The issue seems to be the additional 'ldrb r3, [r3]' added by the
compiler in the broken version.

Best regards

Dirk