[PATCH] MIPS: DEBUG_LL_UART_DIVISOR is 0, use a0 instead

[Ahmad Fatoum]

On 9/11/20 2:33 PM, Du Huanpeng wrote:
> this make it possiable pass a0 as divisor to calculate uart baudrate
> in run time on boot.

This is not Ok. Your last commit has your board pass the divisor in a0 now, but other
boards, like the netgear-wg102, don't. Your change might break their UART because the
code there doesn't expect that a0 needs to have a valid value. If you want to do this,
you should migrate all users of this function all at once to supply a valid divisor
(probably `move a0, DEBUG_LL_UART_DIVISOR`).

It's same thing in C. You don't add a new parameter to a function without checking
that all callsites handle this correctly.

> 
> Signed-off-by: Du Huanpeng <u74147 at gmail.com>
> ---
>  arch/mips/include/asm/debug_ll_ns16550.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/mips/include/asm/debug_ll_ns16550.h b/arch/mips/include/asm/debug_ll_ns16550.h
> index df58c4c..a33587a 100644
> --- a/arch/mips/include/asm/debug_ll_ns16550.h
> +++ b/arch/mips/include/asm/debug_ll_ns16550.h
> @@ -66,6 +66,9 @@ static inline void PUTC_LL(char ch)
>  	sb	t1, UART_LCR(t0)		/* Write it out */
>  
>  	li	t1, DEBUG_LL_UART_DIVISOR
> +#if DEBUG_LL_UART_DIVISOR == 0
> +	move	t1, a0
> +#endif
>  	sb	t1, UART_DLL(t0)		/* write low order byte */
>  	srl	t1, t1, 8
>  	sb	t1, UART_DLM(t0)		/* write high order byte */
> 

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