[U-Boot] [PATCH] PXA: Align stack to 8 bytes

[Eric Miao]

On Sun, Apr 11, 2010 at 3:21 PM, Marek Vasut <marek.vasut at gmail.com> wrote:
> Stack must be aligned to 8 bytes on PXA (possibly all armv5te) for LDRD/STRD
> instructions. In case LDRD/STRD is issued on an unaligned address, the behaviour
> is undefined.
>

Well, I guess this is true for all ARMv5. ARMv6 and later however, provides
support for such unaligned accesses. According to ARM DDI 0100I, A2.8

Prior to ARMv6, doubleword (LDRD/STRD) accesses to memory, where the
address is not doubleword-aligned, are UNPREDICTABLE. Also, data accesses
to non-aligned word and halfword data are treated as aligned from
the memory interface perspective. That is:

• the address is treated as truncated, with address bits[1:0] treated as
  zero for word accesses, and address bit[0] treated as zero for halfword
  accesses.
• load single word ARM instructions are architecturally defined to rotate
  right the word-aligned data transferred by a non word-aligned address
  one, two or three bytes depending on the value of the two least
  significant address bits.
• alignment checking is defined for implementations supporting a System
  Control coprocessor using the A bit in CP15 register 1. When this bit
  is set, a Data Abort indicating an alignment fault is reported for
  unaligned accesses.

ARMv6 introduces unaligned word and halfword load and store data access
support. When this is enabled, the processor uses one or more memory
accesses to generate the required transfer of adjacent bytes transparently
to the programmer, apart from a potential access time penalty where the
transaction crosses an IMPLEMENTATION DEFINED cache-line, bus-width or
page boundary condition. Doubleword accesses must be word-aligned in this
configuration.

> The issue was observed when working with the NAND code, which was rendered
> disfunctional. Also, the vsprintf() function had serious problems with printing
> 64bit wide long longs. After aligning the stack, this wrong behaviour is no
> longer present.
>
> Tested on:
>        Marvell Littleton PXA310 board
>        Toradex Colibri PXA320 board
>        Aeronix Zipit Z2 PXA270 handheld
>        Voipac PXA270 board
>
> Signed-off-by: Marek Vasut <marek.vasut at gmail.com>
> ---
>  cpu/pxa/start.S |    5 ++++-
>  1 files changed, 4 insertions(+), 1 deletions(-)
>
> diff --git a/cpu/pxa/start.S b/cpu/pxa/start.S
> index 13e2edb..bbfcfd1 100644
> --- a/cpu/pxa/start.S
> +++ b/cpu/pxa/start.S
> @@ -140,7 +140,10 @@ stack_setup:
>  #ifdef CONFIG_USE_IRQ
>        sub     r0, r0, #(CONFIG_STACKSIZE_IRQ+CONFIG_STACKSIZE_FIQ)
>  #endif /* CONFIG_USE_IRQ */
> -       sub     sp, r0, #12             /* leave 3 words for abort-stack    */
> +       bic     sp, r0, #7              /* leave 4 words for abort-stack    */
> +                                       /* NOTE: stack MUST be aligned to   */
> +                                       /* 8 bytes in case we want to use   */
> +                                       /* 64bit datatypes (eg. VSPRINTF64) */
>
>  clear_bss:
>        ldr     r0, _bss_start          /* find start of bss segment        */
> --
> 1.7.0
>
>