linux-kernel - [PATCH v7 04/14] firmware_loader: document firmware_sysfs

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Message-Id: <20180510200850.5958-5-mcgrof@kernel.org>
Date:   Thu, 10 May 2018 13:08:40 -0700
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     gregkh@...uxfoundation.org
Cc:     akpm@...ux-foundation.org, keescook@...omium.org,
        josh@...htriplett.org, maco@...roid.com, andy.gross@...aro.org,
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        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
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        "Luis R. Rodriguez" <mcgrof@...nel.org>
Subject: [PATCH v7 04/14] firmware_loader: document firmware_sysfs_fallback()

This also sets the expecations for future fallback interfaces, even
if they are not exported.

Reviewed-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>
---
 drivers/base/firmware_loader/fallback.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/base/firmware_loader/fallback.c b/drivers/base/firmware_loader/fallback.c
index 3db9e0f225ac..9169e7b9800c 100644
--- a/drivers/base/firmware_loader/fallback.c
+++ b/drivers/base/firmware_loader/fallback.c
@@ -662,6 +662,26 @@ static bool fw_run_sysfs_fallback(enum fw_opt opt_flags)
 	return fw_force_sysfs_fallback(opt_flags);
 }
 
+/**
+ * firmware_fallback_sysfs() - use the fallback mechanism to find firmware
+ * @fw: pointer to firmware image
+ * @name: name of firmware file to look for
+ * @device: device for which firmware is being loaded
+ * @opt_flags: options to control firmware loading behaviour
+ * @ret: return value from direct lookup which triggered the fallback mechanism
+ *
+ * This function is called if direct lookup for the firmware failed, it enables
+ * a fallback mechanism through userspace by exposing a sysfs loading
+ * interface. Userspace is in charge of loading the firmware through the syfs
+ * loading interface. This syfs fallback mechanism may be disabled completely
+ * on a system by setting the proc sysctl value ignore_sysfs_fallback to true.
+ * If this false we check if the internal API caller set the @FW_OPT_NOFALLBACK
+ * flag, if so it would also disable the fallback mechanism. A system may want
+ * to enfoce the sysfs fallback mechanism at all times, it can do this by
+ * setting ignore_sysfs_fallback to false and force_sysfs_fallback to true.
+ * Enabling force_sysfs_fallback is functionally equivalent to build a kernel
+ * with CONFIG_FW_LOADER_USER_HELPER_FALLBACK.
+ **/
 int firmware_fallback_sysfs(struct firmware *fw, const char *name,
 			    struct device *device,
 			    enum fw_opt opt_flags,
-- 
2.17.0