[U-Boot] [PATCH] ftsmc020: enhance for features and asm support.

[Macpaul Lin]

HI Wolfgang,

>> This is because writing assembly code (lowlevel_init) is really a
>> necessity for setting the timing
>> and power outpur correctly to these registers (SMC, SDMC, PMU).
>
> What exactly prevents you from writing the very same code in C?
>
>> It is required to give a correct setting to PMU and SMC to make the
>> onboard DRAM works correctly
>> before the code is loaded from ROM to DRAM and then set up stack for C
>> environemnt.

I've traced the start up code of MIPS and POWERPC architecture.
They still have direct access to dram for setting up the small stack
(<4KB) for C environment.

Ex. In the code in mips/start.S
        li      t0, CONFIG_SYS_SDRAM_BASE + CONFIG_SYS_INIT_SP_OFFSET
        la      sp, 0(t0)

        la      t9, board_init_f
        jr      t9
        nop

If you do not have direct ram access, you still need have something
like internal "sram" to set up
this small stack like "arch/powerpc/cpu/mpc5xx/start.S"
        /* Initialize some SPRs that are hard to access from C
         */
        /*----------------------------------------------------------------------*/

        lis     r3, CONFIG_SYS_IMMR at h                   /* Pass IMMR
as arg1 to C routine */
        lis     r2, CONFIG_SYS_INIT_SP_ADDR at h
        ori     r1, r2, CONFIG_SYS_INIT_SP_ADDR at l       /* Set up the
stack in internal SRAM */
        /* Note: R0 is still 0 here */
        stwu    r0, -4(r1)                      /* Clear final stack
frame so that      */
        stwu    r0, -4(r1)                      /* stack backtraces
terminate cleanly   */

However, after I have double confirmed with the hardware and
architecture colleagues.
We do not have both internal SRAM nor have direct access to RAM to
setup this initial stack.

More then that, the initial timing and parameters setting in the SMC
registers for driving the on board DRAM
was not correct for store the initial stack to DRAM.

So it is necessary to set the correct timing to the registers in the
SMC for even just to setup the initial stack
to on board DRAM in lowlevel_init.S.

> We take care to provide global data and an initial stack very, very
> early in the initialization sequence.  You canuse C code long before
> you can access the system RAM.
>
>> Hence assembly code to setting SMC and PMU in lowlevel_init is a necessity

There is not only the work to set SMC in lowlevel_init is enough,
correctly  to set a parameters to PMU is also necessary.
This is because PMU will management the PLL/DLL to generate correct
timing to DRAM, AHB, and other peripheral devices.
The hardware initial value of this PLL/DLL was not correct for help
the DRAM access also.
Even I set the correct timing to SMC (DRAM) was not enough, it is
required to set PMU also.
Hence we need to set the correct value for PMU in lowlevel_init.S.
So there are needs for our board to write asm to set those parameters.

Hope you can understand the above explanations. If you have any
questions please let me know.

-- 
Best regards,
Macpaul Lin