firmware: turris-mox-rwtm: add debugfs documentation

Add debugfs ABI documentation for the ECDSA signatures.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>

diff --git a/Documentation/ABI/testing/debugfs-turris-mox-rwtm b/Documentation/ABI/testing/debugfs-turris-mox-rwtm
new file mode 100644 (file)
index 0000000..2b3255e
--- /dev/null
+++ b/Documentation/ABI/testing/debugfs-turris-mox-rwtm
@@ -0,0 +1,9 @@
+What:          /sys/kernel/debug/turris-mox-rwtm/do_sign
+Date:          Jun 2020
+KernelVersion: 5.8
+Contact:       Marek Behún <marek.behun@nic.cz>
+Description:   (W) Message to sign with the ECDSA private key stored in
+                   device's OTP. The message must be exactly 64 bytes (since
+                   this is intended for SHA-512 hashes).
+               (R) The resulting signature, 136 bytes. This contains the R and
+                   S values of the ECDSA signature, both in big-endian format.