[U-Boot] [PATCH v2] sunxi: Machine id hack to prevent loading buggy sunxi-3.4 kernels

[Siarhei Siamashka]

Right now U-Boot supports the CONFIG_OLD_SUNXI_KERNEL_COMPAT option,
which makes it go out of its way in limiting the selection of PLL clock
frequencies and PMIC voltages in order not to upset outdated buggy
sunxi-3.4 kernel releases. And if the CONFIG_OLD_SUNXI_KERNEL_COMPAT
option is not set, then booting such old kernels exhibits various
failures at runtime. This is very user unfriendly, and there were
already several incidents when people wasted their time being hit
by these runtime failures and trying to debug them.

The right solution is not to add hacks and workarounds to the mainline
U-Boot, but to fix these bugs in the sunxi-3.4 kernel. And in fact,
the updated sunxi-3.4 kernels already exist. Still we need to follow
the 'Principle of Least Surprise' and U-Boot needs to ensure that
the old buggy kernels are not getting happily booted when the
CONFIG_OLD_SUNXI_KERNEL_COMPAT option is not set. And this patch
addresses this particular issue.

This patch makes U-Boot store the 'compatibility revision' number in
the top 4 bits of the machine id and pass it to the kernel. The old
buggy kernels will fail to load with a very much googlable error
message on the serial console (the "r1 = 0x100010bb" part of it):

  "Error: unrecognized/unsupported machine ID (r1 = 0x100010bb)"

This error message can be documented in the linux-sunxi wiki with
proper explanations about how to resolve this situation and where
to get the necessary bugfixes for the sunxi-3.4 kernel.

The fixed sunxi-3.4 kernels implement a revision compatibility check
and clear the top 4 bits of the machine id if everything is alright.
By accepting the machine id with the bits 31:28 set to 1, the sunxi-3.4
kernel effectively certifies that it has the PLL5 clock speed and
AXP209 DCDC3 voltage fixes applied.

It is still possible to set the CONFIG_OLD_SUNXI_KERNEL_COMPAT option
in U-Boot if the user desires to use an outdated unpatched sunxi-3.4
kernel.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka at gmail.com>
---
Changes in v2:
 - Don't break the current behaviour of the CONFIG_OLD_SUNXI_KERNEL_COMPAT
   option
 - Update the commit message with more detailed explanations

 include/configs/sun4i.h        |  4 ++--
 include/configs/sun5i.h        |  4 ++--
 include/configs/sun7i.h        |  4 ++--
 include/configs/sunxi-common.h | 16 ++++++++++++++++
 4 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/include/configs/sun4i.h b/include/configs/sun4i.h
index 87d269b..1537e53 100644
--- a/include/configs/sun4i.h
+++ b/include/configs/sun4i.h
@@ -13,8 +13,6 @@
  */
 #define CONFIG_CLK_FULL_SPEED		1008000000
 
-#define CONFIG_MACH_TYPE		4104
-
 #ifdef CONFIG_USB_EHCI
 #define CONFIG_USB_EHCI_SUNXI
 #define CONFIG_USB_MAX_CONTROLLER_COUNT	2
@@ -25,4 +23,6 @@
  */
 #include <configs/sunxi-common.h>
 
+#define CONFIG_MACH_TYPE	(4104 | ((CONFIG_MACH_TYPE_COMPAT_REV) << 28))
+
 #endif /* __CONFIG_H */
diff --git a/include/configs/sun5i.h b/include/configs/sun5i.h
index 52e3a6f..e755531 100644
--- a/include/configs/sun5i.h
+++ b/include/configs/sun5i.h
@@ -13,8 +13,6 @@
  */
 #define CONFIG_CLK_FULL_SPEED		1008000000
 
-#define CONFIG_MACH_TYPE		4138
-
 #ifdef CONFIG_USB_EHCI
 #define CONFIG_USB_EHCI_SUNXI
 #define CONFIG_USB_MAX_CONTROLLER_COUNT	1
@@ -25,4 +23,6 @@
  */
 #include <configs/sunxi-common.h>
 
+#define CONFIG_MACH_TYPE	(4138 | ((CONFIG_MACH_TYPE_COMPAT_REV) << 28))
+
 #endif /* __CONFIG_H */
diff --git a/include/configs/sun7i.h b/include/configs/sun7i.h
index 7cd7890..f817f73 100644
--- a/include/configs/sun7i.h
+++ b/include/configs/sun7i.h
@@ -14,8 +14,6 @@
  */
 #define CONFIG_CLK_FULL_SPEED		912000000
 
-#define CONFIG_MACH_TYPE		4283
-
 #ifdef CONFIG_USB_EHCI
 #define CONFIG_USB_EHCI_SUNXI
 #define CONFIG_USB_MAX_CONTROLLER_COUNT	2
@@ -31,4 +29,6 @@
  */
 #include <configs/sunxi-common.h>
 
+#define CONFIG_MACH_TYPE	(4283 | ((CONFIG_MACH_TYPE_COMPAT_REV) << 28))
+
 #endif /* __CONFIG_H */
diff --git a/include/configs/sunxi-common.h b/include/configs/sunxi-common.h
index 5ece020..7779b1f 100644
--- a/include/configs/sunxi-common.h
+++ b/include/configs/sunxi-common.h
@@ -13,6 +13,22 @@
 #ifndef _SUNXI_COMMON_CONFIG_H
 #define _SUNXI_COMMON_CONFIG_H
 
+#ifdef CONFIG_OLD_SUNXI_KERNEL_COMPAT
+/*
+ * The U-Boot workarounds bugs in the outdated buggy sunxi-3.4 kernels at the
+ * expense of restricting some features, so the regular machine id values can
+ * be used.
+ */
+# define CONFIG_MACH_TYPE_COMPAT_REV	0
+#else
+/*
+ * A compatibility guard to prevent loading outdated buggy sunxi-3.4 kernels.
+ * Only sunxi-3.4 kernels with appropriate fixes applied are able to pass
+ * beyond the machine id check.
+ */
+# define CONFIG_MACH_TYPE_COMPAT_REV	1
+#endif
+
 /*
  * High Level Configuration Options
  */
-- 
2.0.5