spi: Guarantee cacheline alignment of driver-private data

__spi_alloc_controller() uses a single allocation to accommodate struct
spi_controller and the driver-private data, but places the latter behind
the former.  This order does not guarantee cacheline alignment of the
driver-private data.  (It does guarantee cacheline alignment of struct
spi_controller but the structure doesn't make any use of that property.)

Round up struct spi_controller to cacheline size.  A forthcoming commit
leverages this to grant DMA access to driver-private data of the BCM2835
SPI master.

An alternative, less economical approach would be to use two allocations.

A third approach consists of reversing the order to conserve memory.
But Mark Brown is concerned that it may result in a performance penalty
on architectures that don't like unaligned accesses.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/01625b9b26b93417fb09d2c15ad02dfe9cdbbbe5.1568187525.git.lukas@wunner.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2188,8 +2188,10 @@ extern struct class spi_slave_class;	/* dummy */
  * __spi_alloc_controller - allocate an SPI master or slave controller
  * @dev: the controller, possibly using the platform_bus
  * @size: how much zeroed driver-private data to allocate; the pointer to this
- *	memory is in the driver_data field of the returned device,
- *	accessible with spi_controller_get_devdata().
+ *	memory is in the driver_data field of the returned device, accessible
+ *	with spi_controller_get_devdata(); the memory is cacheline aligned;
+ *	drivers granting DMA access to portions of their private data need to
+ *	round up @size using ALIGN(size, dma_get_cache_alignment()).
  * @slave: flag indicating whether to allocate an SPI master (false) or SPI
  *	slave (true) controller
  * Context: can sleep
@@ -2211,11 +2213,12 @@ struct spi_controller *__spi_alloc_controller(struct device *dev,
 					      unsigned int size, bool slave)
 {
 	struct spi_controller	*ctlr;
+	size_t ctlr_size = ALIGN(sizeof(*ctlr), dma_get_cache_alignment());
 
 	if (!dev)
 		return NULL;
 
-	ctlr = kzalloc(size + sizeof(*ctlr), GFP_KERNEL);
+	ctlr = kzalloc(size + ctlr_size, GFP_KERNEL);
 	if (!ctlr)
 		return NULL;
 
@@ -2229,7 +2232,7 @@ struct spi_controller *__spi_alloc_controller(struct device *dev,
 		ctlr->dev.class = &spi_master_class;
 	ctlr->dev.parent = dev;
 	pm_suspend_ignore_children(&ctlr->dev, true);
-	spi_controller_set_devdata(ctlr, &ctlr[1]);
+	spi_controller_set_devdata(ctlr, (void *)ctlr + ctlr_size);
 
 	return ctlr;
 }