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From:   Martin Kaiser <martin@kaiser.cx>
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     linux-crypto@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, Martin Kaiser <martin@kaiser.cx>
Subject: [PATCH v2 2/2] hwrng: st - keep clock enabled while hwrng is registered
Date:   Fri, 16 Jun 2023 09:58:13 +0100
Message-Id: <20230616085813.1323082-3-martin@kaiser.cx>
In-Reply-To: <20230616085813.1323082-1-martin@kaiser.cx>
References: <20230616083618.1320824-1-martin@kaiser.cx>
 <20230616085813.1323082-1-martin@kaiser.cx>
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The st-rng driver uses devres to register itself with the hwrng core,
the driver will be unregistered from hwrng when its device goes out of
scope. This happens after the driver's remove function is called.

However, st-rng's clock is disabled in the remove function. There's a
short timeframe where st-rng is still registered with the hwrng core
although its clock is disabled. I suppose the clock must be active to
access the hardware and serve requests from the hwrng core.

Switch to devm_clk_get_enabled and let devres disable the clock and
unregister the hwrng. This avoids the race condition.

Fixes: 3e75241be808 ("hwrng: drivers - Use device-managed registration API")
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
---
v2:
  - don't unprepare the clock if devm_hwrng_register fails
  - don't set driver data, it was used only in the remove function that's
    now gone
  - rephrase the commit message, the clock is now disabled by devres

 drivers/char/hw_random/st-rng.c | 21 +--------------------
 1 file changed, 1 insertion(+), 20 deletions(-)

diff --git a/drivers/char/hw_random/st-rng.c b/drivers/char/hw_random/st-rng.c
index 15ba1e6fae4d..6e9dfac9fc9f 100644
--- a/drivers/char/hw_random/st-rng.c
+++ b/drivers/char/hw_random/st-rng.c
@@ -42,7 +42,6 @@
 
 struct st_rng_data {
 	void __iomem	*base;
-	struct clk	*clk;
 	struct hwrng	ops;
 };
 
@@ -85,26 +84,18 @@ static int st_rng_probe(struct platform_device *pdev)
 	if (IS_ERR(base))
 		return PTR_ERR(base);
 
-	clk = devm_clk_get(&pdev->dev, NULL);
+	clk = devm_clk_get_enabled(&pdev->dev, NULL);
 	if (IS_ERR(clk))
 		return PTR_ERR(clk);
 
-	ret = clk_prepare_enable(clk);
-	if (ret)
-		return ret;
-
 	ddata->ops.priv	= (unsigned long)ddata;
 	ddata->ops.read	= st_rng_read;
 	ddata->ops.name	= pdev->name;
 	ddata->base	= base;
-	ddata->clk	= clk;
-
-	dev_set_drvdata(&pdev->dev, ddata);
 
 	ret = devm_hwrng_register(&pdev->dev, &ddata->ops);
 	if (ret) {
 		dev_err(&pdev->dev, "Failed to register HW RNG\n");
-		clk_disable_unprepare(clk);
 		return ret;
 	}
 
@@ -113,15 +104,6 @@ static int st_rng_probe(struct platform_device *pdev)
 	return 0;
 }
 
-static int st_rng_remove(struct platform_device *pdev)
-{
-	struct st_rng_data *ddata = dev_get_drvdata(&pdev->dev);
-
-	clk_disable_unprepare(ddata->clk);
-
-	return 0;
-}
-
 static const struct of_device_id st_rng_match[] __maybe_unused = {
 	{ .compatible = "st,rng" },
 	{},
@@ -134,7 +116,6 @@ static struct platform_driver st_rng_driver = {
 		.of_match_table = of_match_ptr(st_rng_match),
 	},
 	.probe = st_rng_probe,
-	.remove = st_rng_remove
 };
 
 module_platform_driver(st_rng_driver);
-- 
2.30.2