[PATCH v2 2/4] crypto: exynos - Improve performance of PRNG

[Łukasz Stelmach <l.stelmach@xxxxxxxxxxx>]

Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>, Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>

Use memcpy_fromio() instead of custom exynos_rng_copy_random() function
to retrieve generated numbers from the registers of PRNG.

Rearrange the loop around cpu_relax(). In a loop with while() at the
beginning and the cpu_relax() removed the retry variable is decremented
twice (down to 98). This means, the status register is read three
times before the hardware is ready (which is very quick). Apparently,
cpu_relax() requires noticeable amount of time to execute, so it seems
better to call it for the first time before checking the status of
PRNG. The do {} while () loop decrements the retry variable only once,
which means, the time required for cpu_relax() is long enough to for
the PRNG to provide results. On the other hand, performance in this
arrangement isn't much worse than in a loop without cpu_relax().

Signed-off-by: Łukasz Stelmach <l.stelmach@xxxxxxxxxxx>
---
 drivers/crypto/exynos-rng.c | 36 +++++-------------------------------
 1 file changed, 5 insertions(+), 31 deletions(-)

diff --git a/drivers/crypto/exynos-rng.c b/drivers/crypto/exynos-rng.c
index 2f554b82f49f..7d8f658480d3 100644
--- a/drivers/crypto/exynos-rng.c
+++ b/drivers/crypto/exynos-rng.c
@@ -131,34 +131,6 @@ static int exynos_rng_set_seed(struct exynos_rng_dev *rng,
 }
 
 /*
- * Read from output registers and put the data under 'dst' array,
- * up to dlen bytes.
- *
- * Returns number of bytes actually stored in 'dst' (dlen
- * or EXYNOS_RNG_SEED_SIZE).
- */
-static unsigned int exynos_rng_copy_random(struct exynos_rng_dev *rng,
-					   u8 *dst, unsigned int dlen)
-{
-	unsigned int cnt = 0;
-	int i, j;
-	u32 val;
-
-	for (j = 0; j < EXYNOS_RNG_SEED_REGS; j++) {
-		val = exynos_rng_readl(rng, EXYNOS_RNG_OUT(j));
-
-		for (i = 0; i < 4; i++) {
-			dst[cnt] = val & 0xff;
-			val >>= 8;
-			if (++cnt >= dlen)
-				return cnt;
-		}
-	}
-
-	return cnt;
-}
-
-/*
  * Start the engine and poll for finish.  Then read from output registers
  * filling the 'dst' buffer up to 'dlen' bytes or up to size of generated
  * random data (EXYNOS_RNG_SEED_SIZE).
@@ -180,9 +152,10 @@ static int exynos_rng_get_random(struct exynos_rng_dev *rng,
 				  EXYNOS_RNG_SEED_CONF);
 	}
 
-	while (!(exynos_rng_readl(rng,
-			EXYNOS_RNG_STATUS) & EXYNOS_RNG_STATUS_RNG_DONE) && --retry)
+	do {
 		cpu_relax();
+	} while (!(exynos_rng_readl(rng, EXYNOS_RNG_STATUS) &
+		   EXYNOS_RNG_STATUS_RNG_DONE) && --retry);
 
 	if (!retry)
 		return -ETIMEDOUT;
@@ -190,7 +163,8 @@ static int exynos_rng_get_random(struct exynos_rng_dev *rng,
 	/* Clear status bit */
 	exynos_rng_writel(rng, EXYNOS_RNG_STATUS_RNG_DONE,
 			  EXYNOS_RNG_STATUS);
-	*read = exynos_rng_copy_random(rng, dst, dlen);
+	*read = min_t(size_t, dlen, EXYNOS_RNG_SEED_SIZE);
+	memcpy_fromio(dst, rng->mem + EXYNOS_RNG_OUT_BASE, *read);
 
 	return 0;
 }
-- 
2.11.0