On Tue, 8 Sep 2020 14:25:05 +0800 Du Huanpeng <u74147@xxxxxxxxx> wrote: Hi! Adding nop is not necessary. Please see full text of debug_ll_outc_a0: .macro debug_ll_outc_a0 #ifdef CONFIG_DEBUG_LL .set push .set reorder <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< la t0, DEBUG_LL_UART_ADDR 201: lbu t1, UART_LSR(t0) /* get line status */ andi t1, t1, UART_LSR_THRE /* check for transmitter empty */ beqz t1, 201b /* try again */ sb a0, UART_THR(t0) /* write the character */ .set pop #endif /* CONFIG_DEBUG_LL */ .endm There is '.set reorder' so assembler has insert nop after beqz automatically. I have compiled barebox for qemu-malta with DEBUG_LL enabled. barebox$ mips-linux-gnu-objdump -D arch/mips/boards/qemu-malta/lowlevel.pbl.o | grep -A 10 "_nmon_outc_a0>:$" 00000314 <_nmon_outc_a0>: 314: 3c08b800 lui t0,0xb800 318: 350803f8 ori t0,t0,0x3f8 31c: 91090005 lbu t1,5(t0) 320: 31290020 andi t1,t1,0x20 324: 1120fffd beqz t1,31c <_nmon_outc_a0+0x8> 328: 00000000 nop <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< 32c: a1040000 sb a0,0(t0) 330: 03e00008 jr ra 334: 00000000 nop > In the debug_ll_outc_a0 macro, a0 is written to the uart data > register many times in a delay slot when the uart is not ready > to send data. insert a nop to avoid this. > > Signed-off-by: Du Huanpeng <u74147@xxxxxxxxx> > --- > arch/mips/include/asm/debug_ll_ns16550.h | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/mips/include/asm/debug_ll_ns16550.h b/arch/mips/include/asm/debug_ll_ns16550.h > index df58c4c..cf666d1 100644 > --- a/arch/mips/include/asm/debug_ll_ns16550.h > +++ b/arch/mips/include/asm/debug_ll_ns16550.h > @@ -88,6 +88,7 @@ static inline void PUTC_LL(char ch) > 201: lbu t1, UART_LSR(t0) /* get line status */ > andi t1, t1, UART_LSR_THRE /* check for transmitter empty */ > beqz t1, 201b /* try again */ > + nop > > sb a0, UART_THR(t0) /* write the character */ > > -- > 2.7.4 > > > _______________________________________________ > barebox mailing list > barebox@xxxxxxxxxxxxxxxxxxx > http://lists.infradead.org/mailman/listinfo/barebox -- Best regards, Antony Pavlov _______________________________________________ barebox mailing list barebox@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/barebox