Re: [PATCH 2/2] HPET: make frequency calculations 32 bit safe

[Bob Picco <bob.picco@hp.com>]

Clemens Ladisch wrote:	[Thu Sep 22 2005, 11:08:41AM EDT]
> On 32-bit architectures, the multiplication in the argument for
> hpet_time_div() often overflows.  In the typical case of a 14.32 MHz
> timer, this happens when the desired frequency exceeds 61 Hz.
> 
> To avoid this multiplication, we can precompute and store the hardware
> timer frequency, instead of the period, in the device structure, which
> leaves us with a simple division when computing the number of timer
> ticks.
> 
> As a side effect, this also removes a theoretical bug where the timer
> interpolator's frequency would be computed as a 32-bit value even if
> the HPET frequency is greater than 2^32 Hz (the HPET spec allows up to
> 10 GHz).
> 
> Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
> 
> --- linux-2.6.13.orig/drivers/char/hpet.c	2005-09-22 11:10:01.000000000 +0200
> +++ linux-2.6.13/drivers/char/hpet.c	2005-09-22 12:08:48.000000000 +0200
> @@ -78,7 +78,7 @@ struct hpets {
>  	struct hpet __iomem *hp_hpet;
>  	unsigned long hp_hpet_phys;
>  	struct time_interpolator *hp_interpolator;
> -	unsigned long hp_period;
> +	unsigned long long hp_tick_freq;
>  	unsigned long hp_delta;
>  	unsigned int hp_ntimer;
>  	unsigned int hp_which;
> @@ -427,12 +427,14 @@ static int hpet_ioctl_ieon(struct hpet_d
>  	return 0;
>  }
>  
> -static inline unsigned long hpet_time_div(unsigned long dis)
> +/* converts Hz to number of timer ticks */
> +static inline unsigned long hpet_time_div(struct hpets *hpets,
> +					  unsigned long dis)
>  {
> -	unsigned long long m = 1000000000000000ULL;
> +	unsigned long long m;
>  
> +	m = hpets->hp_tick_freq + (dis >> 1);
>  	do_div(m, dis);
> -
>  	return (unsigned long)m;
>  }
>  
> @@ -480,7 +482,7 @@ hpet_ioctl_common(struct hpet_dev *devp,
>  		{
>  			struct hpet_info info;
>  
> -			info.hi_ireqfreq = hpet_time_div(hpetp->hp_period *
> +			info.hi_ireqfreq = hpet_time_div(hpetp,
>  							 devp->hd_ireqfreq);
>  			info.hi_flags =
>  			    readq(&timer->hpet_config) & Tn_PER_INT_CAP_MASK;
> @@ -524,7 +526,7 @@ hpet_ioctl_common(struct hpet_dev *devp,
>  			break;
>  		}
>  
> -		devp->hd_ireqfreq = hpet_time_div(hpetp->hp_period * arg);
> +		devp->hd_ireqfreq = hpet_time_div(hpetp, arg);
>  	}
>  
>  	return err;
> @@ -713,7 +715,7 @@ static void hpet_register_interpolator(s
>  	ti->source = TIME_SOURCE_MMIO64;
>  	ti->shift = 10;
>  	ti->addr = &hpetp->hp_hpet->hpet_mc;
> -	ti->frequency = hpet_time_div(hpets->hp_period);
> +	ti->frequency = hpetp->hp_tick_freq;
>  	ti->drift = ti->frequency * HPET_DRIFT / 1000000;
>  	ti->mask = -1;
>  
> @@ -750,7 +752,7 @@ static unsigned long hpet_calibrate(stru
>  	t = read_counter(&timer->hpet_compare);
>  
>  	i = 0;
> -	count = hpet_time_div(hpetp->hp_period * TICK_CALIBRATE);
> +	count = hpet_time_div(hpetp, TICK_CALIBRATE);
>  
>  	local_irq_save(flags);
>  
> @@ -775,7 +777,8 @@ int hpet_alloc(struct hpet_data *hdp)
>  	size_t siz;
>  	struct hpet __iomem *hpet;
>  	static struct hpets *last = (struct hpets *)0;
> -	unsigned long ns;
> +	unsigned long ns, period;
> +	unsigned long long temp;
>  
>  	/*
>  	 * hpet_alloc can be called by platform dependent code.
> @@ -825,8 +828,12 @@ int hpet_alloc(struct hpet_data *hdp)
>  
>  	last = hpetp;
>  
> -	hpetp->hp_period = (cap & HPET_COUNTER_CLK_PERIOD_MASK) >>
> -	    HPET_COUNTER_CLK_PERIOD_SHIFT;
> +	period = (cap & HPET_COUNTER_CLK_PERIOD_MASK) >>
> +		HPET_COUNTER_CLK_PERIOD_SHIFT; /* fs, 10^-15 */
> +	temp = 1000000000000000uLL; /* 10^15 femtoseconds per second */
> +	temp += period >> 1; /* round */
> +	do_div(temp, period);
> +	hpetp->hp_tick_freq = temp; /* ticks per second */
>  
>  	printk(KERN_INFO "hpet%d: at MMIO 0x%lx, IRQ%s",
>  		hpetp->hp_which, hdp->hd_phys_address,
> @@ -835,8 +842,7 @@ int hpet_alloc(struct hpet_data *hdp)
>  		printk("%s %d", i > 0 ? "," : "", hdp->hd_irq[i]);
>  	printk("\n");
>  
> -	ns = hpetp->hp_period;	/* femptoseconds, 10^-15 */
> -	ns /= 1000000;		/* convert to nanoseconds, 10^-9 */
> +	ns = period / 1000000;	/* convert to nanoseconds, 10^-9 */
>  	printk(KERN_INFO "hpet%d: %ldns tick, %d %d-bit timers\n",
>  		hpetp->hp_which, ns, hpetp->hp_ntimer,
>  		cap & HPET_COUNTER_SIZE_MASK ? 64 : 32);
> -
Sorry for the delay. Looks like my 32 bit code isn't correct for >61 Hz
Never had a 32 bit arch with HPET for testing.  This patch looks fine. 

thanks,

bob
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