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 <[email protected]>: "IIRC, Linus recommends a x2 factor on delays, especially timeouts generated by these functions. Signed-off-by: Lee Jones <[email protected]> --- drivers/char/hw_random/st-rng.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/drivers/char/hw_random/st-rng.c b/drivers/char/hw_random/st-rng.c index 8c8a435..17f0a09 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; -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
