Date: Mon, 12 Jan 2009 13:13:21 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Alexander Clouter <alex@digriz.org.uk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] [REPOST] timer iomem hwrng driver
Message-Id: <20090112131321.b3b8b7b9.akpm@linux-foundation.org>
In-Reply-To: <20081230145209.GA26015@woodchuck>
References: <20081230145209.GA26015@woodchuck>
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On Tue, 30 Dec 2008 14:52:10 +0000
Alexander Clouter <alex@digriz.org.uk> wrote:

> Hi,
> 
> I submitted this some time back but got no 'love' from the community[1] 
> so I'm reposting it.
> 
> Some hardware platforms, the TS-7800[2] is one for example, can supply 
> the kernel with an entropy source, albeit a slow one for TS-7800 users, 
> by just reading a particular IO address.  This source must not be read 
> above a certain rate otherwise the quality is not suitable.
> 
> The driver is then hooked into by calling
> platform_device_(register|add|del) passing a structure similar to: 
> ------------
> #define TS_RNG		(TS78XX_FPGA_REGS_VIRT_BASE | 0x044)
> 
> static struct timeriomem_rng_data ts78xx_ts_rng_data = {
> 	.address	= (u32 *__iomem) TS_RNG,
> 	.period		= 1000000, /* one second */
> };
> 
> static struct platform_device ts78xx_ts_rng_device = {
> 	.name		= "timeriomem_rng",
> 	.id		= -1,
> 	.dev		= {
> 		.platform_data	= &ts78xx_ts_rng_data,
> 	},
> 	.num_resources  = 0,
> };
> ------------
> 

questions...

> +++ b/drivers/char/hw_random/timeriomem-rng.c
> @@ -0,0 +1,153 @@
> +/*
> + * drivers/char/hw_random/timeriomem-rng.c
> + *
> + * Copyright (C) 2008 Alexander Clouter <alex@digriz.org.uk>
> + *
> + * Derived from drivers/char/hw_random/omap-rng.c
> + *   Copyright 2005 (c) MontaVista Software, Inc.
> + *   Author: Deepak Saxena <dsaxena@plexity.net>
> + *
> + * 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.
> + *
> + * Overview:
> + *   This driver is useful for platforms that have an IO range that provides
> + *   periodic random data from a single IO memory address.  All the platform
> + *   has to do is provide the address and 'wait time' that new data becomes
> + *   available.
> + *
> + * TODO: add support for reading sizes other than 32bits and masking
> + */
> +
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include <linux/platform_device.h>
> +#include <linux/hw_random.h>
> +#include <linux/io.h>
> +#include <linux/timeriomem-rng.h>
> +#include <linux/jiffies.h>
> +#include <linux/sched.h>
> +#include <linux/timer.h>
> +
> +static struct timeriomem_rng_data *timeriomem_rng_data;
> +
> +static void timeriomem_rng_trigger(unsigned long);
> +static DEFINE_TIMER(timeriomem_rng_timer, &timeriomem_rng_trigger, 0, 0);
> +
> +/*
> + * have data return 1, however return 0 if we have nothing
> + */
> +static int timeriomem_rng_data_present(struct hwrng *rng, int wait)
> +{
> +	s32 delay;
> +
> +	if (rng->priv == 0)
> +		return 1;
> +
> +	if (timer_pending(&timeriomem_rng_timer)) {
> +		if (!wait)
> +			return 0;
> +
> +		del_timer(&timeriomem_rng_timer);
> +		delay = (long)timeriomem_rng_timer.expires - (long)jiffies;
> +
> +		schedule_timeout_uninterruptible(delay);
> +	}
> +
> +	return 1;
> +}

Would it be better (less racy) to do

	if (del_timer(&timeriomem_rng_timer)) {
		if (!wait)
			return 0;

		delay = (long)timeriomem_rng_timer.expires - (long)jiffies;

		schedule_timeout_uninterruptible(delay);
	}

Secondly, can `delay' be negative, if jiffies increments at just the
right (ie: wrong) time?

Thirdly, why the typecasts in the calculation of `delay'?  Both terms
already have type `unsigned long'.

Fourthly, should it use del_timer_sync()?  Bear in mind that the timer
handler might be concurrently running on another CPU.

> +static int timeriomem_rng_data_read(struct hwrng *rng, u32 *data)
> +{
> +	u32 cur;
> +	s32 delay;
> +
> +	*data = *timeriomem_rng_data->address;

This is reading from I/O memory.  It should use readl()?

> +	if (rng->priv != 0) {
> +		cur = jiffies;
> +
> +		delay = (long)cur - (long)timeriomem_rng_timer.expires;

bug: `cur' should have type `unsigned long'.  The u32 can get truncated.

Then, the casts are unneeded.

> +		delay = rng->priv - (delay % rng->priv);
> +
> +		timeriomem_rng_timer.expires = cur + delay;
> +		add_timer(&timeriomem_rng_timer);
> +	}
> +
> +	return 4;
> +}
> +
> +static void timeriomem_rng_trigger(unsigned long dummy)
> +{
> +	del_timer(&timeriomem_rng_timer);
> +}

del_timer_sync()?

> +static struct hwrng timeriomem_rng_ops = {
> +	.name		= "timeriomem",
> +	.data_present	= timeriomem_rng_data_present,
> +	.data_read	= timeriomem_rng_data_read,
> +	.priv		= 0,
> +};
> +
> +static int __init timeriomem_rng_probe(struct platform_device *pdev)
> +{
> +	int ret;
> +
> +	timeriomem_rng_data = pdev->dev.platform_data;
> +
> +	if (timeriomem_rng_data->period != 0
> +		&& usecs_to_jiffies(timeriomem_rng_data->period) > 0) {
> +		timeriomem_rng_timer.expires = jiffies;
> +		init_timer(&timeriomem_rng_timer);

I don't think the init_timer() is needed - we already (correctly)
initialised it at compile time?

> +		timeriomem_rng_ops.priv = usecs_to_jiffies(
> +						timeriomem_rng_data->period);
> +	}
> +
> +	ret = hwrng_register(&timeriomem_rng_ops);
> +	if (ret) {
> +		dev_err(&pdev->dev, "problem registering\n");
> +		return ret;
> +	}
> +
> +	dev_info(&pdev->dev, "32bits from 0x%p @ %dus\n",
> +			timeriomem_rng_data->address,
> +			timeriomem_rng_data->period);
> +
> +	return 0;
> +}

What will happen if we load this driver on machines which don't
actually have the necessary hardware?  Even non-x86 hardware?

> +static int __devexit timeriomem_rng_remove(struct platform_device *pdev)
> +{
> +	del_timer(&timeriomem_rng_timer);

This should be del_timer_sync().  Otherwise the timer handler could be
running on another CPU during driver teardown.

> +	hwrng_unregister(&timeriomem_rng_ops);
> +
> +	return 0;
> +}
> +
> +static struct platform_driver timeriomem_rng_driver = {
> +	.driver = {
> +		.name		= "timeriomem_rng",
> +		.owner		= THIS_MODULE,
> +	},
> +	.probe		= timeriomem_rng_probe,
> +	.remove		= __devexit_p(timeriomem_rng_remove),
> +};
> +
> +static int __init timeriomem_rng_init(void)
> +{
> +	return platform_driver_register(&timeriomem_rng_driver);
> +}
> +
> +static void __exit timeriomem_rng_exit(void)
> +{
> +	platform_driver_unregister(&timeriomem_rng_driver);
> +}
> +
> +module_init(timeriomem_rng_init);
> +module_exit(timeriomem_rng_exit);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Alexander Clouter <alex@digriz.org.uk>");
> +MODULE_DESCRIPTION("Timer IOMEM H/W RNG driver");
> diff --git a/include/linux/timeriomem-rng.h b/include/linux/timeriomem-rng.h
> new file mode 100644
> index 0000000..16dd9e4
> --- /dev/null
> +++ b/include/linux/timeriomem-rng.h
> @@ -0,0 +1,16 @@
> +/*
> + * linux/include/linux/timeriomem-rng.h
> + *
> + * Copyright (c) 2008 Alexander Clouter <alex@digriz.org.uk>
> + *
> + * 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.
> + */
> +
> +struct timeriomem_rng_data {
> +	u32 __iomem	*address;
> +
> +	/* measures in usecs */
> +	unsigned int	period;
> +};

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