[PATCH v3] x86/earlyprintk: Add a force option for pciserial device

[Feng Tang]

"pciserial" earlyprintk helps much on many modern x86 platforms,
but unfortunately there are still some platforms whose PCI UART
devices have wrong PCI class code, which will be blocked by current
class code check.

Add a option "force" so that developer could still use a UART device
even it has wrong class code, with format "",force,B:D.F,baud". And
the original format ",B:D.F,baud" is kept unchanged.

Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  6 +++++-
 arch/x86/kernel/early_printk.c                  | 26 +++++++++++++++++--------
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 92eb1f4..08b4484 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1063,7 +1063,7 @@
 			earlyprintk=serial[,0x...[,baudrate]]
 			earlyprintk=ttySn[,baudrate]
 			earlyprintk=dbgp[debugController#]
-			earlyprintk=pciserial,bus:device.function[,baudrate]
+			earlyprintk=pciserial[,force],bus:device.function[,baudrate]
 			earlyprintk=xdbc[xhciController#]
 
 			earlyprintk is useful when the kernel crashes before
@@ -1095,6 +1095,10 @@
 
 			The sclp output can only be used on s390.
 
+			The optional "force" for pciserial means insisting to
+			use a PCI device even when its classcode is not of
+			UART class.
+
 	edac_report=	[HW,EDAC] Control how to report EDAC event
 			Format: {"on" | "off" | "force"}
 			on: enable EDAC to report H/W event. May be overridden
diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index 5e801c8..4eb5545 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -213,8 +213,10 @@ static unsigned int mem32_serial_in(unsigned long addr, int offset)
  * early_pci_serial_init()
  *
  * This function is invoked when the early_printk param starts with "pciserial"
- * The rest of the param should be ",B:D.F,baud" where B, D & F describe the
- * location of a PCI device that must be a UART device.
+ * The rest of the param should be ",B:D.F,baud" or ",force,B:D.F,baud", where
+ * B, D & F describe the location of a PCI device that must be a UART device,
+ * "force" is optional and means insisting using a UART device with a wrong
+ * pci class code.
  */
 static __init void early_pci_serial_init(char *s)
 {
@@ -224,17 +226,23 @@ static __init void early_pci_serial_init(char *s)
 	u32 classcode, bar0;
 	u16 cmdreg;
 	char *e;
+	int force = 0;
 
-
-	/*
-	 * First, part the param to get the BDF values
-	 */
 	if (*s == ',')
 		++s;
 
 	if (*s == 0)
 		return;
 
+	/* User may insist to use a UART device with wrong class code */
+	if (!strncmp(s, "force,", 6)) {
+		force = 1;
+		s += 6;
+	}
+
+	/*
+	 * First, part the param to get the BDF values
+	 */
 	bus = (u8)simple_strtoul(s, &e, 16);
 	s = e;
 	if (*s != ':')
@@ -264,8 +272,10 @@ static __init void early_pci_serial_init(char *s)
 	 */
 	if (((classcode >> 16 != PCI_CLASS_COMMUNICATION_MODEM) &&
 	     (classcode >> 16 != PCI_CLASS_COMMUNICATION_SERIAL)) ||
-	   (((classcode >> 8) & 0xff) != 0x02)) /* 16550 I/F at BAR0 */
-		return;
+	   (((classcode >> 8) & 0xff) != 0x02)) /* 16550 I/F at BAR0 */ {
+		if (!force)
+			return;
+	}
 
 	/*
 	 * Determine if it is IO or memory mapped
-- 
2.7.4