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

[John Rigby]

On Tue, Dec 21, 2010 at 1:35 AM, Dirk Behme <dirk.behme at googlemail.com> wrote:
>
> (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 écrit :
>>
>>> 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.
>

And I at your suggestion tried modifying the binary changing the extra
ldrb to a nop and it works.