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

Neil Horman
2006-07-25T17:41:25Z

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");
--
/***************************************************
*Neil Horman
*Software Engineer
*gpg keyid: 1024D / 0x92A74FA1 - http://pgp.mit.edu
***************************************************/