Make an attempt at detecting nonexistent serial port hardware.syslinux-1.64-pre6

4 files changed, 34 insertions, 6 deletions

diff --git a/isolinux.asm b/isolinux.asm
index fdf54de0..2abd748b 100644
--- a/isolinux.asm
+++ b/isolinux.asm
@@ -1222,8 +1222,9 @@ pc_serial:	call getint			; "serial" command
 		cmp di,3
 		ja .port_is_io			; If port > 3 then port is I/O addr
 		shl di,1
-		mov di,[di+serial_base]
-.port_is_io:	mov [SerialPort],di
+		mov di,[di+serial_base]		; Get the I/O port from the BIOS
+.port_is_io:
+		mov [SerialPort],di
 		lea dx,[di+3]			; DX -> LCR
 		mov al,83h			; Enable DLAB
 		call slow_out
@@ -1236,6 +1237,9 @@ pc_serial:	call getint			; "serial" command
 		mov al,03h			; Disable DLAB
 		add dx,byte 2			; DX -> LCR
 		call slow_out
+		in al,dx			; Read back LCR (detect missing hw)
+		cmp al,03h			; If nothing here we'll read 00 or FF
+		jne .serial_port_bad		; Assume serial port busted
 		sub dx,byte 2			; DX -> IER
 		xor al,al			; IRQ disable
 		call slow_out
@@ -1253,6 +1257,10 @@ pc_serial:	call getint			; "serial" command
 
 		jmp short parse_config_3
 
+.serial_port_bad:
+		mov [SerialPort], word 0
+		jmp short parse_config_3
+
 pc_fkey:	sub ah,'1'
 		jnb pc_fkey1
 		mov ah,9			; F10
diff --git a/ldlinux.asm b/ldlinux.asm
index 65b93303..f9552f95 100644
--- a/ldlinux.asm
+++ b/ldlinux.asm
@@ -1320,8 +1320,9 @@ pc_serial:	call getint			; "serial" command
 		cmp di,3
 		ja .port_is_io			; If port > 3 then port is I/O addr
 		shl di,1
-		mov di,[di+serial_base]
-.port_is_io:	mov [SerialPort],di
+		mov di,[di+serial_base]		; Get the I/O port from the BIOS
+.port_is_io:
+		mov [SerialPort],di
 		lea dx,[di+3]			; DX -> LCR
 		mov al,83h			; Enable DLAB
 		call slow_out
@@ -1334,6 +1335,9 @@ pc_serial:	call getint			; "serial" command
 		mov al,03h			; Disable DLAB
 		add dx,byte 2			; DX -> LCR
 		call slow_out
+		in al,dx			; Read back LCR (detect missing hw)
+		cmp al,03h			; If nothing here we'll read 00 or FF
+		jne .serial_port_bad		; Assume serial port busted
 		sub dx,byte 2			; DX -> IER
 		xor al,al			; IRQ disable
 		call slow_out
@@ -1351,6 +1355,10 @@ pc_serial:	call getint			; "serial" command
 
 		jmp short parse_config_3
 
+.serial_port_bad:
+		mov [SerialPort], word 0
+		jmp short parse_config_3
+
 pc_fkey:	sub ah,'1'
 		jnb pc_fkey1
 		mov ah,9			; F10
diff --git a/pxelinux.asm b/pxelinux.asm
index 5a6976d9..85faf63e 100644
--- a/pxelinux.asm
+++ b/pxelinux.asm
@@ -1037,8 +1037,9 @@ pc_serial:	call getint			; "serial" command
 		cmp di,3
 		ja .port_is_io			; If port > 3 then port is I/O addr
 		shl di,1
-		mov di,[di+serial_base]
-.port_is_io:	mov [SerialPort],di
+		mov di,[di+serial_base]		; Get the I/O port from the BIOS
+.port_is_io:
+		mov [SerialPort],di
 		lea dx,[di+3]			; DX -> LCR
 		mov al,83h			; Enable DLAB
 		call slow_out
@@ -1051,6 +1052,9 @@ pc_serial:	call getint			; "serial" command
 		mov al,03h			; Disable DLAB
 		add dx,byte 2			; DX -> LCR
 		call slow_out
+		in al,dx			; Read back LCR (detect missing hw)
+		cmp al,03h			; If nothing here we'll read 00 or FF
+		jne .serial_port_bad		; Assume serial port busted
 		sub dx,byte 2			; DX -> IER
 		xor al,al			; IRQ disable
 		call slow_out
@@ -1068,6 +1072,10 @@ pc_serial:	call getint			; "serial" command
 
 		jmp short parse_config_3
 
+.serial_port_bad:
+		mov [SerialPort], word 0
+		jmp short parse_config_3
+
 pc_fkey:	sub ah,'1'
 		jnb pc_fkey1
 		mov ah,9			; F10
diff --git a/syslinux.doc b/syslinux.doc
index 3a064858..71cbbe4f 100644
--- a/syslinux.doc
+++ b/syslinux.doc
@@ -234,6 +234,10 @@ SERIAL port [[baudrate] flowcontrol]
 	For the SERIAL directive to be guaranteed to work properly, it
 	should be the first directive in the configuration file.
 
+	NOTE: "port" values from 0 to 3 means the first four serial
+	ports detected by the BIOS.  They may or may not correspond to
+	the legacy port values 0x3F8, 0x2F8, 0x3E8, 0x2E8.
+
 FONT filename
 	Load a font in .psf format before displaying any output
 	(except the copyright line, which is output as ldlinux.sys