[PATCH 2/3] hwrng: stm32 - add support for STM32 HW RNG

Hi Daniel,

On 10/03/2015 10:35 PM, Daniel Thompson wrote:

Add support for STMicroelectronics STM32 random number generator.

The config value defaults to N, reflecting the fact that STM32 is a
very low resource microcontroller platform and unlikely to be targeted
by any "grown up" defconfigs.

Signed-off-by: Daniel Thompson <redacted>
---
  drivers/char/hw_random/Kconfig     |  12 +++
  drivers/char/hw_random/Makefile    |   1 +
  drivers/char/hw_random/stm32-rng.c | 192 +++++++++++++++++++++++++++++++++++++
  3 files changed, 205 insertions(+)
  create mode 100644 drivers/char/hw_random/stm32-rng.c

<snip>

<snip>

+#include <linux/clk.h>
+#include <linux/delay.h>
+#include <linux/hw_random.h>
+#include <linux/io.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/of_address.h>
+#include <linux/of_platform.h>
+#include <linux/slab.h>
+
+#define RNG_CR 0x00
+#define RNG_CR_RNGEN BIT(2)
+
+#define RNG_SR 0x04
+#define RNG_SR_SEIS BIT(6)
+#define RNG_SR_CEIS BIT(5)
+#define RNG_SR_DRDY BIT(0)
+
+#define RNG_DR 0x08
+
+/*
+ * It takes 40 cycles @ 48MHz to generate each random number (e.g. <1us).
+ * At the time of writing STM32 parts max out at ~200MHz meaning a timeout
+ * of 500 leaves us a very comfortable margin for error. The loop to which
+ * the timeout applies takes at least 4 instructions per cycle so the
+ * timeout is enough to take us up to multi-GHz parts!
+ */
+#define RNG_TIMEOUT 500
+
+struct stm32_rng_private {
+	struct hwrng rng;
+	void __iomem *base;
+	struct clk *clk;
+};
+
+static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
+{
+	struct stm32_rng_private *priv =
+	    container_of(rng, struct stm32_rng_private, rng);
+	u32 cr, sr;
+	int retval = 0;
+
+	/* enable random number generation */
+	clk_enable(priv->clk);
+	cr = readl(priv->base + RNG_CR);
+	writel(cr | RNG_CR_RNGEN, priv->base + RNG_CR);
+
+	while (max > sizeof(u32)) {
+		sr = readl(priv->base + RNG_SR);
+		if (!sr && wait) {
+			unsigned int timeout = RNG_TIMEOUT;
+
+			do {
+				cpu_relax();
+				sr = readl(priv->base + RNG_SR);
+			} while (!sr && --timeout);
+		}
+
+		/* Has h/ware error dection been triggered? */
+		if (WARN_ON(sr & (RNG_SR_SEIS | RNG_SR_CEIS)))

Maybe you should instead use WARN_ONCE?
Because from what I understand in the datasheet, CEIS bit indicates and 
error with clock configuration.
If that happens, it is likely the same error will occur each time this 
function will be called.

+			break;
+
+		/* No data ready... */
+		if (!sr)
+			break;

Maybe you could perform this check before the error detection, as if 
!sr, the HW error condition will be always false.

+
+		*(u32 *)data = readl(priv->base + RNG_DR);
+
+		retval += sizeof(u32);
+		data += sizeof(u32);
+		max -= sizeof(u32);
+	}
+
+	/* disable the generator */
+	writel(cr, priv->base + RNG_CR);
+	clk_disable(priv->clk);
+
+	return retval || !wait ? retval : -EIO;
+}

Couldn't we use "_relaxed" versions of readl/writel?
I might save some not needed barriers.

+static int stm32_rng_probe(struct platform_device *ofdev)
+{
+	struct device *dev = &ofdev->dev;
+	struct device_node *np = ofdev->dev.of_node;
+	struct stm32_rng_private *priv;
+	struct resource res;
+	int err;
+
+	priv = devm_kzalloc(dev, sizeof(struct stm32_rng_private), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	err = of_address_to_resource(np, 0, &res);
+	if (err)
+		return err;
+
+	priv->base = devm_ioremap_resource(dev, &res);
+	if (IS_ERR(priv->base))
+		return PTR_ERR(priv->base);
+
+	priv->clk = devm_clk_get(&ofdev->dev, NULL);
+	if (IS_ERR(priv->clk))
+		return PTR_ERR(priv->clk);
+
+	dev_set_drvdata(dev, priv);
+
+	priv->rng.name = dev_driver_string(dev),
+	priv->rng.init = stm32_rng_init,
+	priv->rng.cleanup = stm32_rng_cleanup,
+	priv->rng.read = stm32_rng_read,
+	priv->rng.priv = (unsigned long) dev;
+
+	err = hwrng_register(&priv->rng);
+	if (err)
+		return err;
+
+	return 0;

As detected with Coccinelle by Fengguang build system, you could simplify:
return hwrng_register(&priv->rng);

Regards,
Maxime