[PATCH] hwrng: tpm: Do not enable by default

[Jan Kiszka <jan.kiszka@siemens.com>]

From: Jan Kiszka <jan.kiszka@siemens.com>

As seen with optee_ftpm, which uses ms-tpm-20-ref [1], a TPM may write
the current time epoch to its NV storage every 4 seconds if there are
commands sent to it. The 60 seconds periodic update of the entropy pool
that the hwrng kthread does triggers this, causing about 4 writes per
requests. Makes 2 millions per year for a 24/7 device, and that is a lot
for its backing NV storage.

It is therefore better to make the user intentionally enable this,
providing a chance to read the warning.

[1] https://github.com/Microsoft/ms-tpm-20-ref

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
 drivers/char/tpm/Kconfig | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
index 8a8f692b6088..d64c929cacbe 100644
--- a/drivers/char/tpm/Kconfig
+++ b/drivers/char/tpm/Kconfig
@@ -45,13 +45,17 @@ config TCG_TPM2_HMAC
 config HW_RANDOM_TPM
 	bool "TPM HW Random Number Generator support"
 	depends on TCG_TPM && HW_RANDOM && !(TCG_TPM=y && HW_RANDOM=m)
-	default y
 	help
 	  This setting exposes the TPM's Random Number Generator as a hwrng
 	  device. This allows the kernel to collect randomness from the TPM at
 	  boot, and provides the TPM randomines in /dev/hwrng.
 
-	  If unsure, say Y.
+	  WARNING: Specifically firmware-based TPMs, possibly also hardware
+	  variants, can wear-out from the frequent requests issued by the
+	  Hardware Random Number Generator Core when filling the kernel's
+	  entropy pool. These requests are sent once every minute by default,
+	  and the TPM may write the current time to its NV storage for each of
+	  them.
 
 config TCG_TIS_CORE
 	tristate
-- 
2.51.0