Re: [PATCH 2/2] rng: pic32-rng: Add PIC32 RNG hardware driver

[Daniel Thompson <daniel.thompson@linaro.org>]

On 01/02/16 22:41, Joshua Henderson wrote:
> Add support for the hardware pseudo and true random number generator
> peripheral found on PIC32.
>
> Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
> Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
> ---
>   drivers/char/hw_random/Kconfig     |   13 +++
>   drivers/char/hw_random/Makefile    |    1 +
>   drivers/char/hw_random/pic32-rng.c |  152 ++++++++++++++++++++++++++++++++++++
>   3 files changed, 166 insertions(+)
>   create mode 100644 drivers/char/hw_random/pic32-rng.c
>
> diff --git a/drivers/char/hw_random/Kconfig b/drivers/char/hw_random/Kconfig
> index dbf2271..3ab0c46 100644
> --- a/drivers/char/hw_random/Kconfig
> +++ b/drivers/char/hw_random/Kconfig
> @@ -381,6 +381,19 @@ config HW_RANDOM_STM32
>
>   	  If unsure, say N.
>
> +config HW_RANDOM_PIC32
> +	tristate "Microchip PIC32 Random Number Generator support"
> +	depends on HW_RANDOM && MACH_PIC32
> +	default y
> +	---help---
> +	  This driver provides kernel-side support for the Random Number
> +	  Generator hardware found on a PIC32.
> +
> +	  To compile this driver as a module, choose M here. the
> +	  module will be called pic32-rng.
> +
> +	  If unsure, say Y.
> +
>   endif # HW_RANDOM
>
>   config UML_RANDOM
> diff --git a/drivers/char/hw_random/Makefile b/drivers/char/hw_random/Makefile
> index 5ad3976..f5a6fa7 100644
> --- a/drivers/char/hw_random/Makefile
> +++ b/drivers/char/hw_random/Makefile
> @@ -33,3 +33,4 @@ obj-$(CONFIG_HW_RANDOM_MSM) += msm-rng.o
>   obj-$(CONFIG_HW_RANDOM_ST) += st-rng.o
>   obj-$(CONFIG_HW_RANDOM_XGENE) += xgene-rng.o
>   obj-$(CONFIG_HW_RANDOM_STM32) += stm32-rng.o
> +obj-$(CONFIG_HW_RANDOM_PIC32) += pic32-rng.o
> diff --git a/drivers/char/hw_random/pic32-rng.c b/drivers/char/hw_random/pic32-rng.c
> new file mode 100644
> index 0000000..d2f32c4
> --- /dev/null
> +++ b/drivers/char/hw_random/pic32-rng.c
> @@ -0,0 +1,152 @@
> +/*
> + * PIC32 RNG driver
> + *
> + * Joshua Henderson <joshua.henderson@microchip.com>
> + * Copyright (C) 2016 Microchip Technology Inc.  All rights reserved.
> + *
> + * This program is free software; you can distribute it and/or modify it
> + * under the terms of the GNU General Public License (Version 2) as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope it will be useful, but WITHOUT
> + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
> + * for more details.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/delay.h>
> +#include <linux/slab.h>
> +#include <linux/err.h>
> +#include <linux/io.h>
> +#include <linux/hw_random.h>
> +#include <linux/platform_device.h>
> +#include <linux/of.h>
> +#include <linux/of_device.h>
> +#include <linux/clk.h>
> +#include <linux/clkdev.h>
> +
> +#define RNGCON		0x04
> +#define  TRNGEN		BIT(8)
> +#define  PRNGEN		BIT(9)
> +#define  PRNGCONT	BIT(10)
> +#define  TRNGMOD	BIT(11)
> +#define  SEEDLOAD	BIT(12)
> +#define RNGPOLY1	0x08
> +#define RNGPOLY2	0x0C
> +#define RNGNUMGEN1	0x10
> +#define RNGNUMGEN2	0x14
> +#define RNGSEED1	0x18
> +#define RNGSEED2	0x1C
> +#define RNGRCNT		0x20
> +#define  RCNT_MASK	0x7F
> +
> +struct pic32_rng {
> +	void __iomem	*base;
> +	struct hwrng	rng;
> +	struct clk	*clk;
> +};
> +
> +static int pic32_rng_read(struct hwrng *rng, void *buf, size_t max,
> +			  bool wait)
> +{
> +	struct pic32_rng *prng = container_of(rng, struct pic32_rng, rng);
> +	u64 *data = buf;
> +
> +	*data = ((u64)readl(prng->base + RNGNUMGEN2) << 32) +
> +		readl(prng->base + RNGNUMGEN1);
> +	return 4;
> +}

I have a number of questions at this point (although #3 is by far the 
most important).

1. If the random number of 42-bit shouldn't this be return 5; ?

2. If you really do mean return 4; then why mess about with all the
    shifting. You can just read and discard the upper 32-bits.

3. Why are you using the PRNG at all? The PRNG is a simple LSFR and
    I don't think it ever gets reseeded at any point (meaning we get
    almost no useful entropy from it at all).

    Far better to read directly from the TRNG.


> +
> +static int pic32_rng_probe(struct platform_device *pdev)
> +{
> +	struct pic32_rng *prng;
> +	struct resource *res;
> +	u32 v, t;
> +	int ret;
> +
> +	prng = devm_kzalloc(&pdev->dev, sizeof(*prng), GFP_KERNEL);
> +	if (!prng)
> +		return -ENOMEM;
> +
> +	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> +	prng->base = devm_ioremap_resource(&pdev->dev, res);
> +	if (IS_ERR(prng->base))
> +		return PTR_ERR(prng->base);
> +
> +	prng->clk = devm_clk_get(&pdev->dev, NULL);
> +	if (IS_ERR(prng->clk))
> +		return PTR_ERR(prng->clk);
> +
> +	clk_prepare_enable(prng->clk);
> +
> +	/* enable TRNG in enhanced mode */
> +	v = readl(prng->base + RNGCON);
> +	v &= ~(TRNGEN | PRNGEN | 0xff);
> +	v |= TRNGMOD;
> +	writel(v | TRNGEN, prng->base + RNGCON);
> +
> +	/* wait for valid seed */
> +	usleep_range(100, 200);
> +	t = readl(prng->base + RNGRCNT) & RCNT_MASK;
> +	if (t < 0x2A)
> +		dev_warn(&pdev->dev, "seed not generated.\n");
> +
> +	/* load initial seed */
> +	writel(v | SEEDLOAD, prng->base + RNGCON);
> +
> +	/* load initial polynomial: 42bit poly */
> +	writel(0x00c00003, prng->base + RNGPOLY1);
> +	writel(0x00000000, prng->base + RNGPOLY2);
> +
> +	/* start PRNG to generate 42bit random */
> +	v |= 0x2A | PRNGCONT | PRNGEN;
> +	writel(v, prng->base + RNGCON);
> +
> +	prng->rng.name = pdev->name;
> +	prng->rng.read = pic32_rng_read;
> +
> +	ret = hwrng_register(&prng->rng);
> +	if (ret)
> +		goto err_register;
> +
> +	platform_set_drvdata(pdev, prng);
> +
> +	return 0;
> +
> +err_register:
> +	return ret;

Looks like we leak clock references on this error path.


> +
> +static int pic32_rng_remove(struct platform_device *pdev)
> +{
> +	struct pic32_rng *rng = platform_get_drvdata(pdev);
> +
> +	hwrng_unregister(&rng->rng);
> +	writel(0, rng->base + RNGCON);
> +	clk_disable_unprepare(rng->clk);
> +	return 0;
> +}
> +
> +static const struct of_device_id pic32_rng_of_match[] = {
> +	{ .compatible	= "microchip,pic32mzda-rng", },
> +	{ /* sentinel */ }
> +};
> +MODULE_DEVICE_TABLE(of, pic32_rng_of_match);
> +
> +static struct platform_driver pic32_rng_driver = {
> +	.probe		= pic32_rng_probe,
> +	.remove		= pic32_rng_remove,
> +	.driver		= {
> +		.name	= "pic32-rng",
> +		.owner	= THIS_MODULE,
> +		.of_match_table = of_match_ptr(pic32_rng_of_match),
> +	},
> +};
> +
> +module_platform_driver(pic32_rng_driver);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Joshua Henderson <joshua.henderson@microchip.com>");
> +MODULE_DESCRIPTION("Microchip PIC32 RNG Driver");
>