rtc: ds1511: clean up ds1511_nvram_read()/ds1511_nvram_write()

The change removes redundant sysfs binary file boundary checks, since
this task is already done on caller side in fs/sysfs/file.c

The change enables burst mode of access to SRAM for any read()/write()
operations, it is worth to mention that this may influence on
userspace, for instance prior to the change

read(fd, buf, 1);
read(fd, buf + 1, 1);

and

read(fd, buf, 2);

sequences of syscalls over DS1511's sysfs "nvram" fd led to different
DS1511 state changes and/or buf content, if some userspace applications
are written specifically for DS1511 and exploit this strange
"feature", they may be impacted.

Also the change corrects NVRAM size accessible to userspace from 255
bytes to 256 bytes.

Signed-off-by: Vladimir Zapolskiy <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-ds1511.c

···
 #define DS1511_KIE	0x04
 #define DS1511_WDE	0x02
 #define DS1511_WDS	0x01
-#define DS1511_RAM_MAX	0xff
+#define DS1511_RAM_MAX	0x100
 
 #define RTC_CMD		DS1511_CONTROL_B
 #define RTC_CMD1	DS1511_CONTROL_A
···
 	/*
 	 * set wdog enable and wdog 'steering' bit to issue a reset
 	 */
-	rtc_write(DS1511_WDE | DS1511_WDS, RTC_CMD);
+	rtc_write(rtc_read(RTC_CMD) | DS1511_WDE | DS1511_WDS, RTC_CMD);
 }
 
 void
···
 {
 	ssize_t count;
 
-	/*
-	 * if count is more than one, turn on "burst" mode
-	 * turn it off when you're done
-	 */
-	if (size > 1)
-		rtc_write((rtc_read(RTC_CMD) | DS1511_BME), RTC_CMD);
-
-	if (pos > DS1511_RAM_MAX)
-		pos = DS1511_RAM_MAX;
-
-	if (size + pos > DS1511_RAM_MAX + 1)
-		size = DS1511_RAM_MAX - pos + 1;
-
 	rtc_write(pos, DS1511_RAMADDR_LSB);
-	for (count = 0; size > 0; count++, size--)
+	for (count = 0; count < size; count++)
 		*buf++ = rtc_read(DS1511_RAMDATA);
-
-	if (count > 1)
-		rtc_write((rtc_read(RTC_CMD) & ~DS1511_BME), RTC_CMD);
 
 	return count;
 }
···
 {
 	ssize_t count;
 
-	/*
-	 * if count is more than one, turn on "burst" mode
-	 * turn it off when you're done
-	 */
-	if (size > 1)
-		rtc_write((rtc_read(RTC_CMD) | DS1511_BME), RTC_CMD);
-
-	if (pos > DS1511_RAM_MAX)
-		pos = DS1511_RAM_MAX;
-
-	if (size + pos > DS1511_RAM_MAX + 1)
-		size = DS1511_RAM_MAX - pos + 1;
-
 	rtc_write(pos, DS1511_RAMADDR_LSB);
-	for (count = 0; size > 0; count++, size--)
+	for (count = 0; count < size; count++)
 		rtc_write(*buf++, DS1511_RAMDATA);
-
-	if (count > 1)
-		rtc_write((rtc_read(RTC_CMD) & ~DS1511_BME), RTC_CMD);
 
 	return count;
 }
···
 	/*
 	 * turn on the clock and the crystal, etc.
 	 */
-	rtc_write(0, RTC_CMD);
+	rtc_write(DS1511_BME, RTC_CMD);
 	rtc_write(0, RTC_CMD1);
 	/*
 	 * clear the wdog counter