x86/earlyprintk: Add a force option for pciserial device

The "pciserial" earlyprintk variant helps much on many modern x86
platforms, but unfortunately there are still some platforms with PCI
UART devices which have the wrong PCI class code. In that case, the
current class code check does not allow for them to be used for logging.

Add a sub-option "force" which overrides the class code check and thus
the use of such device can be enforced.

 [ bp: massage formulations. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Stuart R . Anderson" <stuart.r.anderson@intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: H Peter Anvin <hpa@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thymo van Beers <thymovanbeers@gmail.com>
Cc: alan@linux.intel.com
Cc: linux-doc@vger.kernel.org
Link: http://lkml.kernel.org/r/20181002164921.25833-1-feng.tang@intel.com

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" to "pciserial" enables use of a
+			PCI device even when its classcode is not of the
+			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

arch/x86/kernel/early_printk.c

@@ -213,8 +213,9 @@ 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 "[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 overrides the use of an UART device with a wrong PCI class code.
  */
 static __init void early_pci_serial_init(char *s)
 {
@@ -224,17 +225,23 @@ 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;
 
+	/* Force the use of an UART device with wrong class code */
+	if (!strncmp(s, "force,", 6)) {
+		force = 1;
+		s += 6;
+	}
+
+	/*
+	 * Part the param to get the BDF values
+	 */
 	bus = (u8)simple_strtoul(s, &e, 16);
 	s = e;
 	if (*s != ':')
@@ -253,7 +260,7 @@ static __init void early_pci_serial_init(char *s)
 		s++;
 
 	/*
-	 * Second, find the device from the BDF
+	 * Find the device from the BDF
 	 */
 	cmdreg = read_pci_config(bus, slot, func, PCI_COMMAND);
 	classcode = read_pci_config(bus, slot, func, PCI_CLASS_REVISION);
@@ -264,8 +271,10 @@ 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
@@ -289,7 +298,7 @@ static __init void early_pci_serial_init(char *s)
 	}
 
 	/*
-	 * Lastly, initialize the hardware
+	 * Initialize the hardware
 	 */
 	if (*s) {
 		if (strcmp(s, "nocfg") == 0)