git: adeec17f8122 - stable/13 - stand: Document that boot0 uses BIOS

The branch stable/13 has been updated by imp:

URL: https://cgit.FreeBSD.org/src/commit/?id=adeec17f8122f9af4940e6e45391b38b377082c1

commit adeec17f8122f9af4940e6e45391b38b377082c1
Author:     Warner Losh <imp@FreeBSD.org>
AuthorDate: 2022-08-26 21:47:21 +0000
Commit:     Warner Losh <imp@FreeBSD.org>
CommitDate: 2023-01-24 21:49:32 +0000

    stand: Document that boot0 uses BIOS
    
    And thus has a limited range of supported baud rates. Also add that
    setting BOOT_BOOT0_COMCONSOLE_SPEED=0 will leave it unchanged which
    sometimes can give you 115200 if the BIOS initialized things outside of
    the normal BIOS baud rates (which many x86 enbedded-targetted boards
    do).
    
    Sponsored by:           Netflix
    Reviewed by:            emaste, manu (earlier versions)
    Suggestions by:         jhb
    Differential Revision:  https://reviews.freebsd.org/D36300
    
    (cherry picked from commit 7ed3228323ef4f9e3130603ea68c3be9c2ed50ce)
---
 stand/i386/boot0/Makefile | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/stand/i386/boot0/Makefile b/stand/i386/boot0/Makefile
index 82eb9eefee76..1453f17751b9 100644
--- a/stand/i386/boot0/Makefile
+++ b/stand/i386/boot0/Makefile
@@ -42,6 +42,13 @@ BOOT_BOOT0_ORG?=	0x600
 ORG=${BOOT_BOOT0_ORG}
 
 # Comm settings for boot0sio.
+#
+# boot0sio uses BIOS INT $0x14 for serial ports, we can only support these
+# baudrates due to INT14's limited interface. In addition, if
+# BOOT_BOOT0_COMCONSOLE_SPEED=0, then the baud rate and frame format will remain
+# unchanged. Some BIOSes initialize the serial ports to 115200, and this may
+# allow boot0sio access at that rate if so.
+#
 # Bit(s) Description
 # 7-5    data rate (110,150,300,600,1200,2400,4800,9600 bps)
 # 4-3    parity (00 or 10 = none, 01 = odd, 11 = even)