[U-Boot] [PATCH v2 2/6] common/cmd_boot: keep ARM v7M in thumb mode during do_go_exec()

Felipe Balbi
2015-04-21T17:47:24Z

On Tue, Apr 21, 2015 at 01:36:54PM -0400, Matt Porter wrote:
> On ARM v7M, the processor will return to ARM mode when executing
> a blx instruction with bit 0 of the address == 0. Always set it

but that's what the 'x' is for, right ? eXchange the CPU mode.

> to 1 to stay in thumb mode.
>
> Signed-off-by: Matt Porter <mporter@konsulko.com>
> ---
> common/cmd_boot.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/common/cmd_boot.c b/common/cmd_boot.c
> index 8f2e070..20ce652 100644
> --- a/common/cmd_boot.c
> +++ b/common/cmd_boot.c
> @@ -38,6 +38,10 @@ static int do_go(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
> * pass address parameter as argv[0] (aka command name),
> * and all remaining args
> */
> +#ifdef CONFIG_CPU_V7M
> + /* For ARM V7M, set bit zero to stay in Thumb mode */
> + addr++;
> +#endif

what if we were in ARM state when we reached this point ? You're now
telling CPU to always switch to Thumb. Is this really what we want ?

From ARM's instruction manual:

"
The BX and BLX instructions can change the processor state from ARM to
Thumb, or from Thumb to ARM.

BLX label always changes the state.

BX Rm and BLX Rm derive the target state from bit[0] of Rm:

if bit[0] of Rm is 0, the processor changes to, or remains in, ARM
state

if bit[0] of Rm is 1, the processor changes to, or remains in, Thumb
state.
"

--
balbi
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