[U-Boot] [Question] Driver-Model UART on NOR-boot ? Work?

[Simon Glass]

Hi Masahiro,

On 1 October 2014 05:23, Masahiro Yamada <yamada.m at jp.panasonic.com> wrote:
> Hi Simon,
>
>
>
> I am looking at the driver-model serial code.
>
>
> I notice driver-model serial code uses ".data" section
> for storing the current device even before relocation.
>
>
> This code in drivers/serial/serial-uclass.c:
>
>   /* The currently-selected console serial device */
>   struct udevice *cur_dev __attribute__ ((section(".data")));
>
>
>
> In my understanding, we should not write any data to
> .data section before relocation.
>
>
> Let's say we are booting U-Boot from NOR flash.
>
> Before relocation, everything (including .data section)
> is placed on NOR flash which is read-only.
> (Please point out if I am wrong.)
>
> We are only allowed to write data to the stack, gd_t, bd_t
> and malloc area (if CONFIG_SYS_MALLOC_F_LEN is defined)
> before relocation, I think.
>
>
>
> I think that is why pre-driver-model serial uses a hard-coded
> default serial port before relocation.
>
>
>   This code in driver/serial/serial.c:
>
>      if (!(gd->flags & GD_FLG_RELOC))
>                 dev = default_serial_console();
>      else if (!serial_current)
>                 dev = default_serial_console();
>      else
>                 dev = serial_current;
>
>
>
>
> My question is,  does printf() work
> with driver-model UART and XIP device such NOR flash?

The global_data structure needs to go somewhere, even before
relocation. On ARM this is general SRAM I suppose. In your case I
think you have some cache RAM to use. Do you use SPL?

The pre-reloc malloc() area goes below global_data so uses the same mechanism.

Yes it is true that strictly speaking we should not use data before
relocation. I suppose we could move cur_dev to global_data instead for
this particular case. It doesn't really scale to other drivers, but
then I expect that only serial needs to keep a 'current' pointer
before relocation, and even that will eventually go away once the
serial stuff is cleaned up.

So let me know if that solution works.

Regards,
Simon