Re: [PATCH] RTC: Add mmap method to rtc character driver

[Jim Gettys <jg@laptop.org>]

Actually, it was Keith Packard who asked for this (and we've asked for
it before in the past).

I will note, that if my memory serves me right, the first X driver we
ever did (1984) had this feature.

                             Regards,
                                       - Jim

("X is an exercise in avoiding system calls.")

P.S. my name is spelled "Gettys".


On Tue, 2006-07-25 at 13:41 -0400, Neil Horman wrote:
> Hey-
> 	At OLS last week, During Dave Jones Userspace Sucks presentation, Jim
> Geddys and some of the Xorg guys noted that they would be able to stop using gettimeofday
> so frequently, if they had some other way to get a millisecond resolution timer
> in userspace, one that they could perhaps read from a memory mapped page.  I was
> right behind them and though that seemed like a reasonable request,  so I've
> taken a stab at it.  This patch allows for a page to be mmaped from /dev/rtc
> character interface, the first 4 bytes of which provide a regularly increasing
> count, once every rtc interrupt.  The frequency is of course controlled by the
> regular ioctls provided by the rtc driver. I've done some basic testing on it,
> and it seems to work well.
> 
> Thanks And Regards
> Neil
> 
> Signed-off-by: Neil Horman
> 
> 
>  
>  rtc.c |   41 ++++++++++++++++++++++++++++++++++++++++-
>  1 files changed, 40 insertions(+), 1 deletion(-)
> 
> 
> diff --git a/drivers/char/rtc.c b/drivers/char/rtc.c
> index 6e6a7c7..4ed673e 100644
> --- a/drivers/char/rtc.c
> +++ b/drivers/char/rtc.c
> @@ -48,9 +48,10 @@
>   *		CONFIG_HPET_EMULATE_RTC
>   *	1.12a	Maciej W. Rozycki: Handle memory-mapped chips properly.
>   *	1.12ac	Alan Cox: Allow read access to the day of week register
> + *	1.12b   Neil Horman: Allow memory mapping of /dev/rtc	
>   */
>  
> -#define RTC_VERSION		"1.12ac"
> +#define RTC_VERSION		"1.12b"
>  
>  /*
>   *	Note that *all* calls to CMOS_READ and CMOS_WRITE are done with
> @@ -183,6 +184,8 @@ static int rtc_proc_open(struct inode *i
>   */
>  static unsigned long rtc_status = 0;	/* bitmapped status byte.	*/
>  static unsigned long rtc_freq = 0;	/* Current periodic IRQ rate	*/
> +#define BUF_SIZE (PAGE_SIZE/sizeof(unsigned long))
> +static unsigned long rtc_irq_buf[BUF_SIZE] __attribute__ ((aligned (PAGE_SIZE)));
>  static unsigned long rtc_irq_data = 0;	/* our output to the world	*/
>  static unsigned long rtc_max_user_freq = 64; /* > this, need CAP_SYS_RESOURCE */
>  
> @@ -230,6 +233,7 @@ static inline unsigned char rtc_is_updat
>  
>  irqreturn_t rtc_interrupt(int irq, void *dev_id, struct pt_regs *regs)
>  {
> +	unsigned long *count_ptr = (unsigned long *)rtc_irq_buf;
>  	/*
>  	 *	Can be an alarm interrupt, update complete interrupt,
>  	 *	or a periodic interrupt. We store the status in the
> @@ -265,6 +269,7 @@ irqreturn_t rtc_interrupt(int irq, void 
>  
>  	kill_fasync (&rtc_async_queue, SIGIO, POLL_IN);
>  
> +	*count_ptr = (*count_ptr)++;
>  	return IRQ_HANDLED;
>  }
>  #endif
> @@ -389,6 +394,37 @@ static ssize_t rtc_read(struct file *fil
>  #endif
>  }
>  
> +static int rtc_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +        unsigned long rtc_addr;
> +	unsigned long *count_ptr = rtc_irq_buf;
> +
> +        if (vma->vm_end - vma->vm_start != PAGE_SIZE)
> +                return -EINVAL;
> +
> +        if (vma->vm_flags & VM_WRITE)
> +                return -EPERM;
> +
> +        if (PAGE_SIZE > (1 << 16))
> +                return -ENOSYS;
> +
> +        vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +
> +        rtc_addr = __pa(rtc_irq_buf);
> +        rtc_addr &= ~(PAGE_SIZE - 1);
> +        rtc_addr &= -1;
> +
> +        if (remap_pfn_range(vma, vma->vm_start, rtc_addr >> PAGE_SHIFT,
> +                                        PAGE_SIZE, vma->vm_page_prot)) {
> +                printk(KERN_ERR "remap_pfn_range failed in rtc.c\n");
> +                return -EAGAIN;
> +        }
> +
> +	*count_ptr = 0;
> +        return 0;
> +
> +}
> +
>  static int rtc_do_ioctl(unsigned int cmd, unsigned long arg, int kernel)
>  {
>  	struct rtc_time wtime; 
> @@ -890,6 +926,7 @@ static const struct file_operations rtc_
>  	.owner		= THIS_MODULE,
>  	.llseek		= no_llseek,
>  	.read		= rtc_read,
> +	.mmap		= rtc_mmap,
>  #ifdef RTC_IRQ
>  	.poll		= rtc_poll,
>  #endif
> @@ -1082,6 +1119,8 @@ no_irq:
>  no_irq2:
>  #endif
>  
> +	memset(rtc_irq_buf,0,PAGE_SIZE);
> +
>  	(void) init_sysctl();
>  
>  	printk(KERN_INFO "Real Time Clock Driver v" RTC_VERSION "\n");
-- 
Jim Gettys
One Laptop Per Child