i2c: core: remove use of in_atomic()

Commit cea443a ("i2c: Support i2c_transfer in atomic contexts")
added in_atomic() to the I2C core. However, the use of in_atomic()
outside of core kernel code is discouraged and was already[1] when this
code was added in early 2008. The above commit was a preparation for
commit b7a3670 ("i2c-pxa: Add polling transfer"). Its commit
message says explicitly it was added "for cases where I2C transactions
have to occur at times interrup[t]s are disabled". So, the intention was
'disabled interrupts'. This matches the use cases for atomic I2C
transfers I have seen so far: very late communication (mostly to a PMIC)
to powerdown or reboot the system. For those cases, interrupts are
disabled then. It doesn't seem that in_atomic() adds value.

After a discussion with Peter Zijlstra[2], we came up with a better set
of conditionals to match the use case.

The I2C core will soon gain an extra callback into bus drivers
especially for atomic transfers to make them more generic. The code
deciding which transfer to use (atomic/non-atomic) should mimic the
behaviour which locking to use (trylock/lock). This is why we add a
helper for it.

[1] https://lwn.net/Articles/274695/
[2] http://patchwork.ozlabs.org/patch/1067437/

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Stefan Lengfeld <contact@stefanchrist.eu>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Author: Wolfram Sang

drivers/i2c/i2c-core-base.c

@@ -1946,7 +1946,7 @@ int i2c_transfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
 	 *    one (discarding status on the second message) or errno
 	 *    (discarding status on the first one).
 	 */
-	if (in_atomic() || irqs_disabled()) {
+	if (i2c_in_atomic_xfer_mode()) {
 		ret = i2c_trylock_bus(adap, I2C_LOCK_SEGMENT);
 		if (!ret)
 			/* I2C activity is ongoing. */

drivers/i2c/i2c-core.h

@@ -29,6 +29,16 @@ extern int		__i2c_first_dynamic_bus_num;
 
 int i2c_check_7bit_addr_validity_strict(unsigned short addr);
 
+/*
+ * We only allow atomic transfers for very late communication, e.g. to send
+ * the powerdown command to a PMIC. Atomic transfers are a corner case and not
+ * for generic use!
+ */
+static inline bool i2c_in_atomic_xfer_mode(void)
+{
+	return system_state > SYSTEM_RUNNING && irqs_disabled();
+}
+
 #ifdef CONFIG_ACPI
 const struct acpi_device_id *
 i2c_acpi_match_device(const struct acpi_device_id *matches,