rtc: fall back to requesting only the ports we actually use

Firmware like PNPBIOS or ACPI can report the address space consumed by the
RTC.  The actual space consumed may be less than the size (RTC_IO_EXTENT)
assumed by the RTC driver.

The PNP core doesn't request resources yet, but I'd like to make it do so.
If/when it does, the RTC_IO_EXTENT request may fail, which prevents the RTC
driver from loading.

Since we only use the RTC index and data registers at RTC_PORT(0) and
RTC_PORT(1), we can fall back to requesting just enough space for those.

If the PNP core requests resources, this results in typical I/O port usage
like this:

    0070-0073 : 00:06		<-- PNP device 00:06 responds to 70-73
      0070-0071 : rtc		<-- RTC driver uses only 70-71

instead of the current:

    0070-0077 : rtc		<-- RTC_IO_EXTENT == 8

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 33 insertions, 6 deletions

diff --git a/drivers/char/rtc.c b/drivers/char/rtc.c
index a162e1b3d5d3..0c66b802736a 100644
--- a/drivers/char/rtc.c
+++ b/drivers/char/rtc.c
@@ -918,12 +918,29 @@ static const struct file_operations rtc_proc_fops = {
 };
 #endif
 
+static resource_size_t rtc_size;
+
+static struct resource * __init rtc_request_region(resource_size_t size)
+{
+        struct resource *r;
+
+        if (RTC_IOMAPPED)
+                r = request_region(RTC_PORT(0), size, "rtc");
+        else
+                r = request_mem_region(RTC_PORT(0), size, "rtc");
+
+        if (r)
+                rtc_size = size;
+
+        return r;
+}
+
 static void rtc_release_region(void)
 {
         if (RTC_IOMAPPED)
-                release_region(RTC_PORT(0), RTC_IO_EXTENT);
+                release_region(RTC_PORT(0), rtc_size);
         else
-                release_mem_region(RTC_PORT(0), RTC_IO_EXTENT);
+                release_mem_region(RTC_PORT(0), rtc_size);
 }
 
 static int __init rtc_init(void)
@@ -976,10 +993,17 @@ found:
         }
 no_irq:
 #else
-        if (RTC_IOMAPPED)
-                r = request_region(RTC_PORT(0), RTC_IO_EXTENT, "rtc");
-        else
-                r = request_mem_region(RTC_PORT(0), RTC_IO_EXTENT, "rtc");
+        r = rtc_request_region(RTC_IO_EXTENT);
+
+        /*
+         * If we've already requested a smaller range (for example, because
+         * PNPBIOS or ACPI told us how the device is configured), the request
+         * above might fail because it's too big.
+         *
+         * If so, request just the range we actually use.
+         */
+        if (!r)
+                r = rtc_request_region(RTC_IO_EXTENT_USED);
         if (!r) {
 #ifdef RTC_IRQ
                 rtc_has_irq = 0;
diff --git a/include/linux/mc146818rtc.h b/include/linux/mc146818rtc.h
index 580b3f4956ee..2f4e957af656 100644
--- a/include/linux/mc146818rtc.h
+++ b/include/linux/mc146818rtc.h
@@ -109,8 +109,11 @@ struct cmos_rtc_board_info {
 #ifndef ARCH_RTC_LOCATION       /* Override by <asm/mc146818rtc.h>? */
 
 #define RTC_IO_EXTENT   0x8
+#define RTC_IO_EXTENT_USED      0x2
 #define RTC_IOMAPPED    1       /* Default to I/O mapping. */
 
+#else
+#define RTC_IO_EXTENT_USED      RTC_IO_EXTENT
 #endif /* ARCH_RTC_LOCATION */
 
 #endif /* _MC146818RTC_H */