Currently one requires to test four kernel configurations to test the
firmware API completely:

0)
  CONFIG_FW_LOADER=y

1)
  o CONFIG_FW_LOADER=y
  o CONFIG_FW_LOADER_USER_HELPER=y

2)
  o CONFIG_FW_LOADER=y
  o CONFIG_FW_LOADER_USER_HELPER=y
  o CONFIG_FW_LOADER_USER_HELPER_FALLBACK=y

3) When CONFIG_FW_LOADER=m the built-in stuff is disabled, we have
   no current tests for this.

We can reduce the requirements to three kernel configurations by making
fw_config.force_sysfs_fallback a proc knob we flip on off. For kernels that
disable CONFIG_IKCONFIG_PROC this can also enable one to inspect if
CONFIG_FW_LOADER_USER_HELPER_FALLBACK was enabled at build time by checking
the proc value at boot time.

Signed-off-by: Luis R. Rodriguez <mcg...@kernel.org>
---
 drivers/base/firmware_fallback.c       |  1 +
 drivers/base/firmware_fallback.h       |  4 +++-
 drivers/base/firmware_fallback_table.c | 17 +++++++++++++++++
 kernel/sysctl.c                        | 11 +++++++++++
 4 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/drivers/base/firmware_fallback.c b/drivers/base/firmware_fallback.c
index 47690207e0ee..cbce9a950cd8 100644
--- a/drivers/base/firmware_fallback.c
+++ b/drivers/base/firmware_fallback.c
@@ -7,6 +7,7 @@
 #include <linux/security.h>
 #include <linux/highmem.h>
 #include <linux/umh.h>
+#include <linux/sysctl.h>
 
 #include "firmware_fallback.h"
 #include "firmware_loader.h"
diff --git a/drivers/base/firmware_fallback.h b/drivers/base/firmware_fallback.h
index 550498c7fa4c..ca7e69a8417b 100644
--- a/drivers/base/firmware_fallback.h
+++ b/drivers/base/firmware_fallback.h
@@ -12,12 +12,14 @@
  *
  * @force_sysfs_fallback: force the sysfs fallback mechanism to be used
  *     as if one had enabled CONFIG_FW_LOADER_USER_HELPER_FALLBACK=y.
+ *     Useful to help debug a CONFIG_FW_LOADER_USER_HELPER_FALLBACK=y
+ *     functionality on a kernel where that config entry has been disabled.
  * @old_timeout: for internal use
  * @loading_timeout: the timeout to wait for the fallback mechanism before
  *     giving up, in seconds.
  */
 struct firmware_fallback_config {
-       const bool force_sysfs_fallback;
+       unsigned int force_sysfs_fallback;
        int old_timeout;
        int loading_timeout;
 };
diff --git a/drivers/base/firmware_fallback_table.c 
b/drivers/base/firmware_fallback_table.c
index 53cc4e492520..77300d5e9c52 100644
--- a/drivers/base/firmware_fallback_table.c
+++ b/drivers/base/firmware_fallback_table.c
@@ -19,6 +19,9 @@
 /* Module or buit-in */
 #ifdef CONFIG_FW_LOADER_USER_HELPER
 
+static unsigned int zero;
+static unsigned int one = 1;
+
 struct firmware_fallback_config fw_fallback_config = {
        .force_sysfs_fallback = 
IS_ENABLED(CONFIG_FW_LOADER_USER_HELPER_FALLBACK),
        .loading_timeout = 60,
@@ -26,4 +29,18 @@ struct firmware_fallback_config fw_fallback_config = {
 };
 EXPORT_SYMBOL_GPL(fw_fallback_config);
 
+struct ctl_table firmware_config_table[] = {
+       {
+               .procname       = "force_sysfs_fallback",
+               .data           = &fw_fallback_config.force_sysfs_fallback,
+               .maxlen         = sizeof(unsigned int),
+               .mode           = 0644,
+               .proc_handler   = proc_douintvec_minmax,
+               .extra1         = &zero,
+               .extra2         = &one,
+       },
+       { }
+};
+EXPORT_SYMBOL_GPL(firmware_config_table);
+
 #endif
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index acdf4e85c0a1..aa8355953fcf 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -253,6 +253,10 @@ extern struct ctl_table random_table[];
 extern struct ctl_table epoll_table[];
 #endif
 
+#ifdef CONFIG_FW_LOADER_USER_HELPER
+extern struct ctl_table firmware_config_table[];
+#endif
+
 #ifdef HAVE_ARCH_PICK_MMAP_LAYOUT
 int sysctl_legacy_va_layout;
 #endif
@@ -748,6 +752,13 @@ static struct ctl_table kern_table[] = {
                .mode           = 0555,
                .child          = usermodehelper_table,
        },
+#ifdef CONFIG_FW_LOADER_USER_HELPER
+       {
+               .procname       = "firmware_config",
+               .mode           = 0555,
+               .child          = firmware_config_table,
+       },
+#endif
        {
                .procname       = "overflowuid",
                .data           = &overflowuid,
-- 
2.16.2

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