i2c: core: introduce callbacks for atomic transfers

We had the request to access devices very late when interrupts are not
available anymore multiple times now. Mostly to prepare shutdown or
reboot. Allow adapters to specify a specific callback for this case.
Note that we fall back to the generic {master|smbus}_xfer callback if
this new atomic one is not present. This is intentional to preserve the
previous behaviour and avoid regressions. Because there are drivers not
using interrupts or because it might have worked "accidently" before.

Signed-off-by: Wolfram Sang <[e-mail]>
Reviewed-by Andy Shevchenko <[e-mail]>
Tested-by: Stefan Lengfeld <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

drivers/i2c/i2c-core-base.c

···
 	/* Retry automatically on arbitration loss */
 	orig_jiffies = jiffies;
 	for (ret = 0, try = 0; try <= adap->retries; try++) {
-		ret = adap->algo->master_xfer(adap, msgs, num);
+		if (i2c_in_atomic_xfer_mode() && adap->algo->master_xfer_atomic)
+			ret = adap->algo->master_xfer_atomic(adap, msgs, num);
+		else
+			ret = adap->algo->master_xfer(adap, msgs, num);
+
 		if (ret != -EAGAIN)
 			break;
 		if (time_after(jiffies, orig_jiffies + adap->timeout))

drivers/i2c/i2c-core-smbus.c

···
 		     unsigned short flags, char read_write,
 		     u8 command, int protocol, union i2c_smbus_data *data)
 {
+	int (*xfer_func)(struct i2c_adapter *adap, u16 addr,
+			 unsigned short flags, char read_write,
+			 u8 command, int size, union i2c_smbus_data *data);
 	unsigned long orig_jiffies;
 	int try;
 	s32 res;
···
 
 	flags &= I2C_M_TEN | I2C_CLIENT_PEC | I2C_CLIENT_SCCB;
 
-	if (adapter->algo->smbus_xfer) {
+	xfer_func = adapter->algo->smbus_xfer;
+	if (i2c_in_atomic_xfer_mode()) {
+		if (adapter->algo->smbus_xfer_atomic)
+			xfer_func = adapter->algo->smbus_xfer_atomic;
+		else if (adapter->algo->master_xfer_atomic)
+			xfer_func = NULL; /* fallback to I2C emulation */
+	}
+
+	if (xfer_func) {
 		/* Retry automatically on arbitration loss */
 		orig_jiffies = jiffies;
 		for (res = 0, try = 0; try <= adapter->retries; try++) {
-			res = adapter->algo->smbus_xfer(adapter, addr, flags,
-							read_write, command,
-							protocol, data);
+			res = xfer_func(adapter, addr, flags, read_write,
+					command, protocol, data);
 			if (res != -EAGAIN)
 				break;
 			if (time_after(jiffies,

drivers/i2c/i2c-core.h

···
 {
 	int ret = 0;
 
-	if (i2c_in_atomic_xfer_mode())
+	if (i2c_in_atomic_xfer_mode()) {
+		WARN(!adap->algo->master_xfer_atomic && !adap->algo->smbus_xfer_atomic,
+		     "No atomic I2C transfer handler for '%s'\n", dev_name(&adap->dev));
 		ret = i2c_trylock_bus(adap, I2C_LOCK_SEGMENT) ? 0 : -EAGAIN;
-	else
+	} else {
 		i2c_lock_bus(adap, I2C_LOCK_SEGMENT);
+	}
 
 	return ret;
 }

include/linux/i2c.h

···
  * @master_xfer: Issue a set of i2c transactions to the given I2C adapter
  *   defined by the msgs array, with num messages available to transfer via
  *   the adapter specified by adap.
+ * @master_xfer_atomic: same as @master_xfer. Yet, only using atomic context
+ *   so e.g. PMICs can be accessed very late before shutdown. Optional.
  * @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
  *   is not present, then the bus layer will try and convert the SMBus calls
  *   into I2C transfers instead.
+ * @smbus_xfer_atomic: same as @smbus_xfer. Yet, only using atomic context
+ *   so e.g. PMICs can be accessed very late before shutdown. Optional.
  * @functionality: Return the flags that this algorithm/adapter pair supports
  *   from the I2C_FUNC_* flags.
  * @reg_slave: Register given client to I2C slave mode of this adapter
···
  * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
  * to name two of the most common.
  *
- * The return codes from the @master_xfer field should indicate the type of
- * error code that occurred during the transfer, as documented in the kernel
- * Documentation file Documentation/i2c/fault-codes.
+ * The return codes from the @master_xfer{_atomic} fields should indicate the
+ * type of error code that occurred during the transfer, as documented in the
+ * Kernel Documentation file Documentation/i2c/fault-codes.
  */
 struct i2c_algorithm {
 	/*
···
 	 */
 	int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
 			   int num);
+	int (*master_xfer_atomic)(struct i2c_adapter *adap,
+				   struct i2c_msg *msgs, int num);
 	int (*smbus_xfer)(struct i2c_adapter *adap, u16 addr,
 			  unsigned short flags, char read_write,
 			  u8 command, int size, union i2c_smbus_data *data);
+	int (*smbus_xfer_atomic)(struct i2c_adapter *adap, u16 addr,
+				 unsigned short flags, char read_write,
+				 u8 command, int size, union i2c_smbus_data *data);
 
 	/* To determine what the adapter supports */
 	u32 (*functionality)(struct i2c_adapter *adap);