[PATCH 3/3] ARM: early_printk: use printascii() rather than printch()

[Nicolas Pitre]

On Tue, 31 Oct 2017, Robin Murphy wrote:

> On 31/10/17 16:22, Nicolas Pitre wrote:
> > On Tue, 31 Oct 2017, Chris Brandt wrote:
> > 
> >> On Sunday, October 01, 2017 1, Nicolas Pitre wrote:
> >>> With printch() the console messages are sent out one character at a time
> >>> which is agonizingly slow especially with semihosting as the whole trap
> >>> intercept, remote byte access, and system resume danse is performed for
> >>> every single character across a relatively slow remote debug connection.
> >>> Let's use printascii() to send a whole string at once. This is also going
> >>> to be more efficient, albeit to a quite lesser extent, with serial ports
> >>> as well.
> >>>
> >>> Signed-off-by: Nicolas Pitre <nico at linaro.org>
> >>> ---
> >>>  arch/arm/kernel/early_printk.c | 16 ++++++++++------
> >>>  1 file changed, 10 insertions(+), 6 deletions(-)
> >>
> >> Now that this patch has hit -next, I'm noticing an issue with it.
> >>
> >> There are no carriage returns, just line feeds, which makes for a very 
> >> ugly display.
> > 
> > Hmmm....
> > 
> > If you look at printascii in arch/arm/kernel/debug.S you'll find the 
> > following code:
> > 
> > 1:              waituart r2, r3
> >                 senduart r1, r3
> >                 busyuart r2, r3
> >                 teq     r1, #'\n'
> >                 moveq   r1, #'\r'
> >                 beq     1b
> > 
> > Why is that not working for you?
> 
> By inspection, the removed early_write() code inserted the '\r' before
> the '\n' in the usual fashion; the printascii() code above ends up doing
> the reverse, and I can imagine the atypical "\n\r" sequence probably
> confuses some terminals trying to be clever with line ending detection.

That's easy to veryfy with this patch:

diff --git a/arch/arm/kernel/debug.S b/arch/arm/kernel/debug.S
index ea9646cc2a..40023a4871 100644
--- a/arch/arm/kernel/debug.S
+++ b/arch/arm/kernel/debug.S
@@ -79,18 +79,21 @@ hexbuf:		.space 16
 
 ENTRY(printascii)
 		addruart_current r3, r1, r2
-		b	2f
-1:		waituart r2, r3
-		senduart r1, r3
-		busyuart r2, r3
-		teq	r1, #'\n'
-		moveq	r1, #'\r'
-		beq	1b
-2:		teq	r0, #0
+1:		teq	r0, #0
 		ldrneb	r1, [r0], #1
 		teqne	r1, #0
-		bne	1b
-		ret	lr
+		reteq	lr
+		teq     r1, #'\n'
+		bne	2f
+		mov	r1, '\r'
+		waituart r2, r3
+		senduart r1, r3
+		busyuart r2, r3
+		mov	r1, '\n'
+2:		waituart r2, r3
+		senduart r1, r3
+		busyuart r2, r3
+		b	1b
 ENDPROC(printascii)
 
 ENTRY(printch)

@Chris: does the above fix your display issue?


Nicolas