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authorLee Jones <lee.jones@linaro.org>2015-10-07 15:23:28 +0300
committerHerbert Xu <herbert@gondor.apana.org.au>2015-10-08 16:42:24 +0300
commit93d649bd94e1003e2a766a59fadba0cc0089d58b (patch)
tree7fb8674ff57d4e9f2c59d902be96dfe0ffeab551 /drivers/char/hw_random
parent8fd80f8ca60bdeda4f3098986e8a566ba9c0c57a (diff)
downloadlinux-93d649bd94e1003e2a766a59fadba0cc0089d58b.tar.xz
hwrng: st - Use real-world device timings for timeout
Samples are documented to be available every 0.667us, so in theory the 8 sample deep FIFO should take 5.336us to fill. However, during thorough testing, it became apparent that filling the FIFO actually takes closer to 12us. Also take into consideration that udelay() can behave oddly i.e. not delay for as long as requested. Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>: "IIRC, Linus recommends a x2 factor on delays, especially timeouts generated by these functions. Signed-off-by: Lee Jones <lee.jones@linaro.org> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Diffstat (limited to 'drivers/char/hw_random')
-rw-r--r--drivers/char/hw_random/st-rng.c10
1 files changed, 8 insertions, 2 deletions
diff --git a/drivers/char/hw_random/st-rng.c b/drivers/char/hw_random/st-rng.c
index 8c8a435176e8..17f0a092bf24 100644
--- a/drivers/char/hw_random/st-rng.c
+++ b/drivers/char/hw_random/st-rng.c
@@ -32,8 +32,14 @@
#define ST_RNG_FIFO_SIZE 8
#define ST_RNG_SAMPLE_SIZE 2 /* 2 Byte (16bit) samples */
-/* Samples are available every 0.667us, which we round to 1us */
-#define ST_RNG_FILL_FIFO_TIMEOUT (1 * (ST_RNG_FIFO_SIZE / ST_RNG_SAMPLE_SIZE))
+/*
+ * Samples are documented to be available every 0.667us, so in theory
+ * the 4 sample deep FIFO should take 2.668us to fill. However, during
+ * thorough testing, it became apparent that filling the FIFO actually
+ * takes closer to 12us. We then multiply by 2 in order to account for
+ * the lack of udelay()'s reliability, suggested by Russell King.
+ */
+#define ST_RNG_FILL_FIFO_TIMEOUT (12 * 2)
struct st_rng_data {
void __iomem *base;