[U-Boot-Users] CFG_INIT_DCACHE_CS

[Stefan Roese]

Hello Moayuan,

> Now I am trying to port ppcboot to a simple custom board,
> with ppc405cr, 32M RAM, 4M EEPROM, JTAG, UART.

Welcome!

> I can not understand some Marcos in the header file include/configs/xxx.h.
>
> #define CFG_INIT_DCACHE_CS      4      /*  use cs # 4 for data cache
> memory    */
>
> there is no OCM in 405CR, so ppcboot needs data cache for initial stacks
> and data.
> but what does the "chip  select" mean?
>
> As I know, chip select should be used on some peripheral devices, like
> Flash, NVRAM,
> why is it needed for data cache which is inside the CPU core?
>
> Is the CS only a concept inside ppcboot?     or  something according to
> the  hardware?

You are right, the PPC405CR doesn't have the OCM, so the data cache has to
be used for the initial stack. The Chip-Select is required to specify a
memory region, so the PPC doesn't generate "bus-errors" upon accessing this
cached memory region. Please use an unconnected CS (if possible) for this
setup.

Best regards,
Stefan.