Re: BeagleBone Black doesn't boot without serial console (10.0 release)

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Fri, Oct 11, 2024 at 01:54:18PM +0200, Martin Husemann wrote:
> On Fri, Oct 11, 2024 at 01:23:21PM +0200, Christoph Badura wrote:
> > What he meant was that he removed the console cable.
> 
> And the reconnecting of the console probably caused a "break", making   
> the kernel enter ddb.
> 
> Since with a 3 wire serial connection there is no way for the hardware to
> detect presence or abscence of the connection it is unclear what happened
> while the console cable was disconnected.

From the schematic here:
https://github.com/beagleboard/BeagleBone-Black/blob/master/BBB_SCH.pdf

there is a pulldown resitor on UART0_RX input. This means that without
a (powered up) USB/serial adapter, UART0_RX sees a logical 0 which
is most probably interpreted as a BREAK by the AM335x UART, and causes
the kernel to enter DDB.

The solution is to either rebuild a kernel without DDB, or tie UART0_RX to
a +3.3V pin.
Or patch the kernel sources like this:

Index: com.c
===================================================================
RCS file: /cvsroot/src/sys/dev/ic/com.c,v
retrieving revision 1.382
diff -u -p -u -r1.382 com.c
--- com.c	9 Dec 2022 00:35:58 -0000	1.382
+++ com.c	11 Oct 2024 12:33:45 -0000
@@ -2647,7 +2647,8 @@ comcnattach1(struct com_regs *regsp, int
 
 	cn_tab = &comcons;
 	cn_init_magic(&com_cnm_state);
-	cn_set_magic("\047\001"); /* default magic is BREAK */
+	// cn_set_magic("\047\001"); /* default magic is BREAK */
+	cn_set_magic("+++++"); 
 
 	comcons_info.frequency = frequency;
 	comcons_info.type = type;
@@ -2734,7 +2735,8 @@ com_kgdb_attach1(struct com_regs *regsp,
 		 * expects this to be initialized
 		 */
 		cn_init_magic(&com_cnm_state);
-		cn_set_magic("\047\001");
+		// cn_set_magic("\047\001");
+		cn_set_magic("+++++");
 	}
 
 	kgdb_attach(com_kgdb_getc, com_kgdb_putc, NULL);

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--