[PATCH] hw_random: bcm2835: use hwrng_msleep() instead of cpu_relax()

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Mark Brown <broonie@kernel.org>,
	Florian Fainelli <f.fainelli@gmail.com>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	kernelci-results@groups.io, bot@kernelci.org,
	gtucker@collabora.com, linux-arm-kernel@lists.infradead.org,
	linux-rpi-kernel@lists.infradead.org,
	bcm-kernel-feedback-list@broadcom.com,
	linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] hw_random: bcm2835: use hwrng_msleep() instead of cpu_relax()
Date: Mon, 10 Oct 2022 09:06:07 -0600	[thread overview]
Message-ID: <20221010150607.720600-1-Jason@zx2c4.com> (raw)
In-Reply-To: <Y0QjkjU4bsOHWFOd@sirena.org.uk>

Rather than busy looping, yield back to the scheduler and sleep for a
bit in the event that there's no data. This should hopefully prevent the
stalls that Mark reported:

<6>[    3.362859] Freeing initrd memory: 16196K
<3>[   23.160131] rcu: INFO: rcu_sched self-detected stall on CPU
<3>[   23.166057] rcu:  0-....: (2099 ticks this GP) idle=03b4/1/0x40000002 softirq=28/28 fqs=1050
<4>[   23.174895]       (t=2101 jiffies g=-1147 q=2353 ncpus=4)
<4>[   23.180203] CPU: 0 PID: 49 Comm: hwrng Not tainted 6.0.0 #1
<4>[   23.186125] Hardware name: BCM2835
<4>[   23.189837] PC is at bcm2835_rng_read+0x30/0x6c
<4>[   23.194709] LR is at hwrng_fillfn+0x71/0xf4
<4>[   23.199218] pc : [<c07ccdc8>]    lr : [<c07cb841>]    psr: 40000033
<4>[   23.205840] sp : f093df70  ip : 00000000  fp : 00000000
<4>[   23.211404] r10: c3c7e800  r9 : 00000000  r8 : c17e6b20
<4>[   23.216968] r7 : c17e6b64  r6 : c18b0a74  r5 : c07ccd99  r4 : c3f171c0
<4>[   23.223855] r3 : 000fffff  r2 : 00000040  r1 : c3c7e800  r0 : c3f171c0
<4>[   23.230743] Flags: nZcv  IRQs on  FIQs on  Mode SVC_32  ISA Thumb  Segment none
<4>[   23.238426] Control: 50c5387d  Table: 0020406a  DAC: 00000051
<4>[   23.244519] CPU: 0 PID: 49 Comm: hwrng Not tainted 6.0.0 #1

Link: https://lore.kernel.org/all/Y0QJLauamRnCDUef@sirena.org.uk/
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
I haven't tested this. Somebody with access to that kernel CI infra that
triggered this will need to test.

 drivers/char/hw_random/bcm2835-rng.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/bcm2835-rng.c b/drivers/char/hw_random/bcm2835-rng.c
index e7dd457e9b22..e98fcac578d6 100644
--- a/drivers/char/hw_random/bcm2835-rng.c
+++ b/drivers/char/hw_random/bcm2835-rng.c
@@ -71,7 +71,7 @@ static int bcm2835_rng_read(struct hwrng *rng, void *buf, size_t max,
 	while ((rng_readl(priv, RNG_STATUS) >> 24) == 0) {
 		if (!wait)
 			return 0;
-		cpu_relax();
+		hwrng_msleep(rng, 1000);
 	}
 
 	num_words = rng_readl(priv, RNG_STATUS) >> 24;
-- 
2.37.3


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