crypto: caam - change starting entropy delay value

The entropy delay (the length in system clocks of each
entropy sample) for the RNG4 block of CAAM is dependent
on the frequency of the SoC. By elaborate methods, it
has been determined that a good starting value for all
platforms integrating the CAAM IP is 3200. Using a
higher value has additional benefit of  speeding up
the process of instantiating the RNG, since the entropy
delay will be increased and instantiation of the RNG
state handles will be reattempted by the driver. If the
starting value is low, for certain platforms, this can
lead to a quite lengthy process.
This patch changes the starting value of the length of
the entropy sample to 3200 system clocks.
In addition to this change, the attempted entropy delay
values are now printed on the console upon initialization
of the RNG block.
While here, a safeguard for yielding the processor was
added for ensuring that in very adverse cases,
the CPU isn't hogged by the instantiation loop.
Signed-off-by: Alex Porosanu <alexandru.porosanu@freescale.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/caam/ctrl.c

@@ -544,6 +544,9 @@ static int caam_probe(struct platform_device *pdev)
 			 * the TRNG parameters.
 			 */
 			if (!(ctrlpriv->rng4_sh_init || inst_handles)) {
+				dev_info(dev,
+					 "Entropy delay = %u\n",
+					 ent_delay);
 				kick_trng(pdev, ent_delay);
 				ent_delay += 400;
 			}
@@ -556,6 +559,12 @@ static int caam_probe(struct platform_device *pdev)
 			 */
 			ret = instantiate_rng(dev, inst_handles,
 					      gen_sk);
+			if (ret == -EAGAIN)
+				/*
+				 * if here, the loop will rerun,
+				 * so don't hog the CPU
+				 */
+				cpu_relax();
 		} while ((ret == -EAGAIN) && (ent_delay < RTSDCTL_ENT_DLY_MAX));
 		if (ret) {
 			dev_err(dev, "failed to instantiate RNG");

drivers/crypto/caam/regs.h

@@ -278,7 +278,7 @@ struct rng4tst {
 	};
 #define RTSDCTL_ENT_DLY_SHIFT 16
 #define RTSDCTL_ENT_DLY_MASK (0xffff << RTSDCTL_ENT_DLY_SHIFT)
-#define RTSDCTL_ENT_DLY_MIN 1200
+#define RTSDCTL_ENT_DLY_MIN 3200
 #define RTSDCTL_ENT_DLY_MAX 12800
 	u32 rtsdctl;		/* seed control register */
 	union {