Re: [PATCH 4/9] omap_uart: add low level port serial initialization

[Sascha Hauer <s.hauer@pengutronix.de>]

On Sun, Mar 10, 2013 at 11:16:00PM +0100, vj wrote:
> On Sun, Mar 10, 2013 at 2:16 PM, Jean-Christophe PLAGNIOL-VILLARD
> <plagnioj@jcrosoft.com> wrote:
> > On 00:19 Sun 10 Mar     , Vicente Bergas wrote:
> >>  some sort of UART setup has to be done in order to use PUTC_LL
> >>
> >> Signed-off-by: Vicente Bergas <vicencb@gmail.com>
> >> ---
> >>  arch/arm/mach-omap/include/mach/debug_ll.h | 21 +++++++++++++++++++++
> >>  1 file changed, 21 insertions(+)
> >>
> >> diff --git a/arch/arm/mach-omap/include/mach/debug_ll.h b/arch/arm/mach-omap/include/mach/debug_ll.h
> >> index eea6eb3..9740848 100644
> >> --- a/arch/arm/mach-omap/include/mach/debug_ll.h
> >> +++ b/arch/arm/mach-omap/include/mach/debug_ll.h
> >> @@ -45,9 +45,30 @@
> >>  #endif
> >>
> >>  #define LSR_THRE     0x20    /* Xmit holding register empty */
> >> +#define LCR_BKSE     0x80    /* Bank select enable */
> >>  #define LSR          (5 << 2)
> >>  #define THR          (0 << 2)
> >> +#define DLL          (0 << 2)
> >> +#define IER          (1 << 2)
> >> +#define DLM          (1 << 2)
> >> +#define FCR          (2 << 2)
> >> +#define LCR          (3 << 2)
> >> +#define MCR          (4 << 2)
> >> +#define MDR          (8 << 2)
> >>
> >> +static inline void INIT_LL(void)
> > where this come from?
> 
> This comes from drivers/serial/serial_ns16550.c: ns16550_serial_init_port
> ns16550_serial_init_port call other functions and has local variables,
> so a stack is required
> INIT_LL instead is an extremly simple inline function without local
> variables, so it can be called at the very beginning
> 
> from include/debug_ll.h:
> "Note that several SoCs expect the UART to be initialized by a
> debugger or a first stage bootloader."
> INIT_LL implements the second option.
> 
> I have a question regarding code executed at the very beginning, that
> is, when the stack may not be available or the binary is not at it's
> link address
> What are the things that can't be done in such scenario?
> For example, from include/debug_ll.h:
> "Be careful with PUTS_LL, it only works if the binary is running at
> the link address which often is not the case during early startup."
> static __inline__ void PUTS_LL(const char * str)
> {
> 	while (*str) {
> 		if (*str == '\n') {
> 			PUTC_LL('\r');
> 		}
> 		PUTC_LL(*str);
> 		str++;
> 	}
> }
> What does PUTS_LL do that can't be done?

You can't access global variables at that state. They are
addressed with their absolute address which will not be correct
when you are not running at the linked address. You can simply
verify that by doing a PUTHEX_LL("somestring"); You will see
the address the string is located when running from the correct
address.

Sascha

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