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Message-ID: <20151005121102.GB17172@x1>
Date: Mon, 5 Oct 2015 13:11:02 +0100
From: Lee Jones <lee.jones@...aro.org>
To: Daniel Thompson <daniel.thompson@...aro.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
devicetree@...r.kernel.org, herbert@...dor.apana.org.au,
linux-crypto@...r.kernel.org, peter@...sgaard.com,
festevam@...il.com, kieranbingham@...il.com, kernel@...inux.com,
Pankaj Dev <pankaj.dev@...com>
Subject: Re: [PATCH v2 5/7] hwrng: st: Add support for ST's HW Random Number
Generator
On Mon, 05 Oct 2015, Daniel Thompson wrote:
> Late but...
That's okay. Fixup patches can always be submitted.
We have forever. :)
> On 17/09/15 14:45, Lee Jones wrote:
> >diff --git a/drivers/char/hw_random/Makefile b/drivers/char/hw_random/Makefile
> >index 055bb01..8bcfb45 100644
> >--- a/drivers/char/hw_random/Makefile
> >+++ b/drivers/char/hw_random/Makefile
> >@@ -30,4 +30,5 @@ obj-$(CONFIG_HW_RANDOM_TPM) += tpm-rng.o
> > obj-$(CONFIG_HW_RANDOM_BCM2835) += bcm2835-rng.o
> > obj-$(CONFIG_HW_RANDOM_IPROC_RNG200) += iproc-rng200.o
> > obj-$(CONFIG_HW_RANDOM_MSM) += msm-rng.o
> >+obj-$(CONFIG_HW_RANDOM_ST) += st-rng.o
> > obj-$(CONFIG_HW_RANDOM_XGENE) += xgene-rng.o
> >diff --git a/drivers/char/hw_random/st-rng.c b/drivers/char/hw_random/st-rng.c
> >new file mode 100644
> >index 0000000..8c8a435
> >--- /dev/null
> >+++ b/drivers/char/hw_random/st-rng.c
> >@@ -0,0 +1,144 @@
> >+/*
> >+ * ST Random Number Generator Driver ST's Platforms
> >+ *
> >+ * Author: Pankaj Dev: <pankaj.dev@...com>
> >+ * Lee Jones <lee.jones@...aro.org>
> >+ *
> >+ * Copyright (C) 2015 STMicroelectronics (R&D) Limited
> >+ *
> >+ * This program is free software; you can redistribute it and/or modify
> >+ * it under the terms of the GNU General Public License version 2 as
> >+ * published by the Free Software Foundation.
> >+ */
> >+
> >+#include <linux/clk.h>
> >+#include <linux/delay.h>
> >+#include <linux/hw_random.h>
> >+#include <linux/io.h>
> >+#include <linux/module.h>
> >+#include <linux/of.h>
> >+#include <linux/platform_device.h>
> >+#include <linux/slab.h>
> >+
> >+/* Registers */
> >+#define ST_RNG_STATUS_REG 0x20
> >+#define ST_RNG_DATA_REG 0x24
> >+
> >+/* Registers fields */
> >+#define ST_RNG_STATUS_BAD_SEQUENCE BIT(0)
> >+#define ST_RNG_STATUS_BAD_ALTERNANCE BIT(1)
> >+#define ST_RNG_STATUS_FIFO_FULL BIT(5)
> >+
> >+#define ST_RNG_FIFO_SIZE 8
> >+#define ST_RNG_SAMPLE_SIZE 2 /* 2 Byte (16bit) samples */
> >+
> >+/* Samples are available every 0.667us, which we round to 1us */
> >+#define ST_RNG_FILL_FIFO_TIMEOUT (1 * (ST_RNG_FIFO_SIZE / ST_RNG_SAMPLE_SIZE))
> >+
> >+struct st_rng_data {
> >+ void __iomem *base;
> >+ struct clk *clk;
> >+ struct hwrng ops;
> >+};
> >+
> >+static int st_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
> >+{
> >+ struct st_rng_data *ddata = (struct st_rng_data *)rng->priv;
> >+ u32 status;
> >+ int i;
> >+
> >+ if (max < sizeof(u16))
> >+ return -EINVAL;
> >+
> >+ /* Wait until FIFO is full - max 4uS*/
> >+ for (i = 0; i < ST_RNG_FILL_FIFO_TIMEOUT; i++) {
> >+ status = readl_relaxed(ddata->base + ST_RNG_STATUS_REG);
> >+ if (status & ST_RNG_STATUS_FIFO_FULL)
> >+ break;
> >+ udelay(1);
>
> How much bandwidth does using udelay() cost? I think it could be
> >10% compared to a tighter polling loop.
Samples are only available every 0.7uS and we only do this for every
4. The maximum it could 'cost' is <1uS. Do we really want to fuss
over that tiny amount of time? It's an understandable point if we
were talking about milliseconds, but a single microsecond?
> >+ }
> >+
> >+ if (i == ST_RNG_FILL_FIFO_TIMEOUT)
> >+ return 0;
>
> Isn't a timeout an error condition?
Yes, which is why we're returning 0. In this context 0 == 'no data'.
This will be converted to -EAGAIN if the caller did not request
NONBLOCKING.
> >+
> >+ for (i = 0; i < ST_RNG_FIFO_SIZE && i < max; i += 2)
> >+ *(u16 *)(data + i) =
> >+ readl_relaxed(ddata->base + ST_RNG_DATA_REG);
> >+
> >+ return i; /* No of bytes read */
> >+}
> >+
> >+static int st_rng_probe(struct platform_device *pdev)
> >+{
> >+ struct st_rng_data *ddata;
> >+ struct resource *res;
> >+ struct clk *clk;
> >+ void __iomem *base;
> >+ int ret;
> >+
> >+ ddata = devm_kzalloc(&pdev->dev, sizeof(*ddata), GFP_KERNEL);
> >+ if (!ddata)
> >+ return -ENOMEM;
> >+
> >+ res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> >+ base = devm_ioremap_resource(&pdev->dev, res);
> >+ if (IS_ERR(base))
> >+ return PTR_ERR(base);
> >+
> >+ clk = devm_clk_get(&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 = hwrng_register(&ddata->ops);
> >+ if (ret) {
> >+ dev_err(&pdev->dev, "Failed to register HW RNG\n");
>
> Why shout about this particular error but not any others? Perhaps
> just rely on the driver core to report the error here?
I have omitted error prints from subsystem calls which do all the
shouting required. Unfortunately the HWRNG is somewhat stuck in the
past in a number of ways; a lack of subsystem level shouting being one
of them.
> >+ return ret;
> >+ }
> >+
> >+ dev_info(&pdev->dev, "Successfully registered HW RNG\n");
> >+
> >+ return 0;
> >+}
> >+
> >+static int st_rng_remove(struct platform_device *pdev)
> >+{
> >+ struct st_rng_data *ddata = dev_get_drvdata(&pdev->dev);
> >+
> >+ hwrng_unregister(&ddata->ops);
> >+
> >+ clk_disable_unprepare(ddata->clk);
>
> This mismatches the error paths in the probe function (there is no
> cleanup of clock counts in probe function).
Good catch. I am missing a clk_disable_unprepare() in the
hwrng_register() failure patch. Will fix.
--
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog
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