On Mon, 14 Oct 2019 at 17:57, Daniel Thompson daniel.thompson@linaro.org wrote:

On Mon, Oct 14, 2019 at 05:32:45PM +0530, Sumit Garg wrote:

...

Existing RNG data read timeout is 200us but it doesn't cover EIP76 RNG data rate which takes approx. 700us to produce 16 bytes of output data as per testing results. So configure the timeout as 1000us to also take account of lack of udelay()'s reliability.

What "lack of udelay()'s reliability" are you concerned about?

For this I took reference from "drivers/char/hw_random/st-rng.c +33". I think it's probably safe to take additional timeout rather than relying on accuracy of udelay() based measurements. Specifically if udelay() returns early than the expected delay (see: include/linux/delay.h).

-Sumit

Daniel.

...

Fixes: 383212425c92 ("hwrng: omap - Add device variant for SafeXcel IP-76 found in Armada 8K") Cc: stable@vger.kernel.org Signed-off-by: Sumit Garg sumit.garg@linaro.org

---

drivers/char/hw_random/omap-rng.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/omap-rng.c b/drivers/char/hw_random/omap-rng.c index b27f396..e329f82 100644 --- a/drivers/char/hw_random/omap-rng.c +++ b/drivers/char/hw_random/omap-rng.c @@ -66,6 +66,13 @@ #define OMAP4_RNG_OUTPUT_SIZE 0x8 #define EIP76_RNG_OUTPUT_SIZE 0x10

+/*

• • EIP76 RNG takes approx. 700us to produce 16 bytes of output data
• • as per testing results. And to account for the lack of udelay()'s
• • reliability, we keep the timeout as 1000us.
• */

+#define RNG_DATA_FILL_TIMEOUT 100

enum { RNG_OUTPUT_0_REG = 0, RNG_OUTPUT_1_REG, @@ -176,7 +183,7 @@ static int omap_rng_do_read(struct hwrng *rng, void *data, size_t max, if (max < priv->pdata->data_size) return 0;

• for (i = 0; i < 20; i++) {
  

• for (i = 0; i < RNG_DATA_FILL_TIMEOUT; i++) {
          present = priv->pdata->data_present(priv);
          if (present || !wait)
                  break;
  

-- 2.7.4