[U-Boot] [PATCH 1/2 V2] arm926: Flush the data cache before disabling it.

Christian Riesch
2012-01-13T08:06:26Z

Hi Sughosh,
I had a look at the patch and I tried to understand what's going on
here (I must confess that I didn't know anything about this cache
stuff).

On Tue, Jan 10, 2012 at 7:12 PM, Sughosh Ganu <urwithsughosh@gmail.com> wrote:
> The current implementation invalidates the cache instead of flushing
> it. This causes problems on platforms where the spl/u-boot is already
> loaded to the RAM, with caches enabled by a first stage bootloader.
>
> The V bit of the cp15's control register c1 is set to the value of
> VINITHI on reset. Do not clear this bit by default, as there are SOC's
> with no valid memory region at 0x0.
[...]
> diff --git a/arch/arm/cpu/arm926ejs/start.S b/arch/arm/cpu/arm926ejs/start.S
> index 6a09c02..6e261c2 100644
> --- a/arch/arm/cpu/arm926ejs/start.S
> +++ b/arch/arm/cpu/arm926ejs/start.S
> @@ -355,17 +355,20 @@ _dynsym_start_ofs:
> ?*/
> ?cpu_init_crit:
> ? ? ? ?/*
> - ? ? ? ?* flush v4 I/D caches
> + ? ? ? ?* flush D cache before disabling it
> ? ? ? ? */
> ? ? ? ?mov ? ? r0, #0
> - ? ? ? mcr ? ? p15, 0, r0, c7, c7, 0 ? /* flush v3/v4 cache */
> +flush_dcache:
> + ? ? ? mrc ? ? p15, 0, r15, c7, c10, 3
> + ? ? ? bne ? ? flush_dcache
> +

Ok, instead of invalidating the D-cache you are cleaning it. From the
ARM926EJ-S Technical Reference Manual [1]: "To guarantee that memory
coherency is maintained, the DCache must be cleaned of dirty data
before it is disabled." So since we are disabling D-Cache a few lines
later (bic r0, r0, #0x00000087), this must be the right way to do it.

> ? ? ? ?mcr ? ? p15, 0, r0, c8, c7, 0 ? /* flush v4 TLB */
>
> ? ? ? ?/*
> ? ? ? ? * disable MMU stuff and caches
> ? ? ? ? */
> ? ? ? ?mrc ? ? p15, 0, r0, c1, c0, 0
> - ? ? ? bic ? ? r0, r0, #0x00002300 ? ? /* clear bits 13, 9:8 (--V- --RS) */
> + ? ? ? bic ? ? r0, r0, #0x00000300 ? ? /* clear bits 9:8 ( --RS) */

Ok, I read your comment above.

> ? ? ? ?bic ? ? r0, r0, #0x00000087 ? ? /* clear bits 7, 2:0 (B--- -CAM) */
> ? ? ? ?orr ? ? r0, r0, #0x00000002 ? ? /* set bit 2 (A) Align */
> ? ? ? ?orr ? ? r0, r0, #0x00001000 ? ? /* set bit 12 (I) I-Cache */

Although this is not changed in your patch, the last line makes me
wonder. The comment says "disable MMU stuff and cached", but actually
the last line sets bit 12 (I), which means that I-Cache gets enabled
according to [1].

Regards, Christian

[1] http://infocenter.arm.com/help/index.jsp