[PATCH] crypto: atmel-sha204a - drop hwrng quality reduction for ATSHA204A

[Thorsten Blum <thorsten.blum@linux.dev>]

Commit 8006aff15516 ("crypto: atmel-sha204a - Set hwrng quality to
lowest possible") reduced the hwrng quality to 1 based on a review by
Bill Cox [1]. However, despite its title, the review only tested the
ATSHA204, not the ATSHA204A.

In the same thread, Atmel engineer Landon Cox wrote "this behavior has
been eliminated entirely"[2] in the ATSHA204A and "this problem does not
affect the ATECC108 or the ATECC108A (or the ATSHA204A)"[3].

According to the official ATSHA204A datasheet [4], the device contains a
high-quality hardware RNG that combines its output with an internal seed
value stored in EEPROM or SRAM to generate random numbers. The device
also implements all security functions using SHA-256, and the driver
uses the chip's Random command in seed-update mode.

Keep 'quality = 1' for ATSHA204, but drop the explicit hwrng quality
reduction for ATSHA204A and fall back to the hwrng core default.

[1] https://www.metzdowd.com/pipermail/cryptography/2014-December/023858.html
[2] https://www.metzdowd.com/pipermail/cryptography/2014-December/023852.html
[3] https://www.metzdowd.com/pipermail/cryptography/2014-December/023886.html
[4] https://ww1.microchip.com/downloads/en/DeviceDoc/ATSHA204A-Data-Sheet-40002025A.pdf

Fixes: 8006aff15516 ("crypto: atmel-sha204a - Set hwrng quality to lowest possible")
Cc: stable@vger.kernel.org
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
 drivers/crypto/atmel-sha204a.c | 40 ++++++++++++++++++----------------
 1 file changed, 21 insertions(+), 19 deletions(-)

diff --git a/drivers/crypto/atmel-sha204a.c b/drivers/crypto/atmel-sha204a.c
index dbb39ed0cea1..df69fb190e52 100644
--- a/drivers/crypto/atmel-sha204a.c
+++ b/drivers/crypto/atmel-sha204a.c
@@ -19,6 +19,25 @@
 #include <linux/workqueue.h>
 #include "atmel-i2c.h"
 
+enum atmel_sha204a_variant {
+	ATSHA204 = 1,
+	ATSHA204A,
+};
+
+static const struct of_device_id atmel_sha204a_dt_ids[] __maybe_unused = {
+	{ .compatible = "atmel,atsha204",  .data = (void *)ATSHA204 },
+	{ .compatible = "atmel,atsha204a", .data = (void *)ATSHA204A },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, atmel_sha204a_dt_ids);
+
+static const struct i2c_device_id atmel_sha204a_id[] = {
+	{ .name = "atsha204",  .driver_data = ATSHA204 },
+	{ .name = "atsha204a", .driver_data = ATSHA204A },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(i2c, atmel_sha204a_id);
+
 static void atmel_sha204a_rng_done(struct atmel_i2c_work_data *work_data,
 				   void *areq, int status)
 {
@@ -171,11 +190,8 @@ static int atmel_sha204a_probe(struct i2c_client *client)
 	i2c_priv->hwrng.name = dev_name(&client->dev);
 	i2c_priv->hwrng.read = atmel_sha204a_rng_read;
 
-	/*
-	 * According to review by Bill Cox [1], this HWRNG has very low entropy.
-	 * [1] https://www.metzdowd.com/pipermail/cryptography/2014-December/023858.html
-	 */
-	i2c_priv->hwrng.quality = 1;
+	if ((uintptr_t)i2c_get_match_data(client) == ATSHA204)
+		i2c_priv->hwrng.quality = 1;
 
 	ret = devm_hwrng_register(&client->dev, &i2c_priv->hwrng);
 	if (ret)
@@ -202,20 +218,6 @@ static void atmel_sha204a_remove(struct i2c_client *client)
 	kfree((void *)i2c_priv->hwrng.priv);
 }
 
-static const struct of_device_id atmel_sha204a_dt_ids[] __maybe_unused = {
-	{ .compatible = "atmel,atsha204", },
-	{ .compatible = "atmel,atsha204a", },
-	{ /* sentinel */ }
-};
-MODULE_DEVICE_TABLE(of, atmel_sha204a_dt_ids);
-
-static const struct i2c_device_id atmel_sha204a_id[] = {
-	{ "atsha204" },
-	{ "atsha204a" },
-	{ /* sentinel */ }
-};
-MODULE_DEVICE_TABLE(i2c, atmel_sha204a_id);
-
 static struct i2c_driver atmel_sha204a_driver = {
 	.probe			= atmel_sha204a_probe,
 	.remove			= atmel_sha204a_remove,