[PATCH v2] crypto: atmel-sha204a - drop hwrng quality reduction for ATSHA204A

Thorsten Blum
2026-04-28T10:15:37Z

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>
---
Changes in v2:
- Drop the enum and dereference match data if needed as suggested by Ard
- Keep the review comment
- v1: https://lore.kernel.org/lkml/20260427124030.315590-3-thorsten.blum@linux.dev/
---
drivers/crypto/atmel-sha204a.c | 19 ++++++++++++-------
1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/crypto/atmel-sha204a.c b/drivers/crypto/atmel-sha204a.c
index dbb39ed0cea1..a8c1b00b12f5 100644
--- a/drivers/crypto/atmel-sha204a.c
+++ b/drivers/crypto/atmel-sha204a.c
@@ -19,6 +19,12 @@
#include <linux/workqueue.h>
#include "atmel-i2c.h"

+/*
+ * According to review by Bill Cox [1], the ATSHA204 has very low entropy.
+ * [1] https://www.metzdowd.com/pipermail/cryptography/2014-December/023858.html
+ */
+static const unsigned short atsha204_quality = 1;
+
static void atmel_sha204a_rng_done(struct atmel_i2c_work_data *work_data,
void *areq, int status)
{
@@ -158,6 +164,7 @@ static const struct attribute_group atmel_sha204a_groups = {
static int atmel_sha204a_probe(struct i2c_client *client)
{
struct atmel_i2c_client_priv *i2c_priv;
+ const unsigned short *quality;
int ret;

ret = atmel_i2c_probe(client);
@@ -171,11 +178,9 @@ 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;
+ quality = i2c_get_match_data(client);
+ if (quality)
+ i2c_priv->hwrng.quality = *quality;

ret = devm_hwrng_register(&client->dev, &i2c_priv->hwrng);
if (ret)
@@ -203,14 +208,14 @@ static void atmel_sha204a_remove(struct i2c_client *client)
}

static const struct of_device_id atmel_sha204a_dt_ids[] __maybe_unused = {
- { .compatible = "atmel,atsha204", },
+ { .compatible = "atmel,atsha204", .data = &atsha204_quality },
{ .compatible = "atmel,atsha204a", },
{ /* sentinel */ }
};
MODULE_DEVICE_TABLE(of, atmel_sha204a_dt_ids);

static const struct i2c_device_id atmel_sha204a_id[] = {
- { "atsha204" },
+ { "atsha204", (kernel_ulong_t)&atsha204_quality },
{ "atsha204a" },
{ /* sentinel */ }
};