[PATCH v4] usb: core: setup dma_pfn_offset for USB devices and, interfaces

[Roger Quadros]

If dma_pfn_offset is not inherited correctly from the host controller,
it might result in sub-optimal configuration as bounce
buffer limit might be set to less than optimal level.

Consider the mass storage device case.
USB storage driver creates a scsi host for the mass storage interface in
drivers/usb/storage/usb.c
The scsi host parent device is nothing but the the USB interface device.
Now, __scsi_init_queue() calls scsi_calculate_bounce_limit() to find out
and set the block layer bounce limit.
scsi_calculate_bounce_limit() uses dma_max_pfn(host_dev) to get the
bounce_limit. host_dev is nothing but the device representing the
mass storage interface.
If that device doesn't have the right dma_pfn_offset, then dma_max_pfn()
is messed up and the bounce buffer limit is wrong.

e.g. On Keystone 2 systems, dma_max_pfn() is 0x87FFFF and dma_mask_pfn
is 0xFFFFF. Consider a mass storage use case: Without this patch,
usb scsi host device (usb-storage) will get a dma_pfn_offset of 0 resulting
in a dma_max_pfn() of 0xFFFFF within the scsi layer
(scsi_calculate_bounce_limit()).
This will result in bounce buffers being unnecessarily used.

Hint: On 32-bit ARM platforms dma_max_pfn() = dma_mask_pfn + dma_pfn_offset

Signed-off-by: Roger Quadros <rogerq@xxxxxx>
---
Changelog:

v4:
- added comment in the code as to why we need to set dma_mask and
dma_pfn_offset for usb devices.
v3:
- removed comments from code as commit log is sufficient.
v2:
- added more information in commit log and code.

 drivers/usb/core/message.c |  5 +++++
 drivers/usb/core/usb.c     | 11 +++++++++++
 2 files changed, 16 insertions(+)

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 0406a59..bb617df 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -1862,7 +1862,12 @@ free_interfaces:
 		intf->dev.bus = &usb_bus_type;
 		intf->dev.type = &usb_if_device_type;
 		intf->dev.groups = usb_interface_groups;
+		/*
+		 * Please refer to usb_alloc_dev() to see why we set
+		 * dma_mask and dma_pfn_offset.
+		 */
 		intf->dev.dma_mask = dev->dev.dma_mask;
+		intf->dev.dma_pfn_offset = dev->dev.dma_pfn_offset;
 		INIT_WORK(&intf->reset_ws, __usb_queue_reset_device);
 		intf->minor = -1;
 		device_initialize(&intf->dev);
diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
index 5e80697..5921514 100644
--- a/drivers/usb/core/usb.c
+++ b/drivers/usb/core/usb.c
@@ -440,7 +440,18 @@ struct usb_device *usb_alloc_dev(struct usb_device *parent,
 	dev->dev.bus = &usb_bus_type;
 	dev->dev.type = &usb_device_type;
 	dev->dev.groups = usb_device_groups;
+	/*
+	 * Fake a dma_mask/offset for the USB device:
+	 * We cannot really use the dma-mapping API (dma_alloc_* and
+	 * dma_map_*) for USB devices but instead need to use
+	 * usb_alloc_coherent and pass data in 'urb's, but some subsystems
+	 * manually look into the mask/offset pair to determine whether
+	 * they need bounce buffers.
+	 * Note: calling dma_set_mask() on a USB device would set the
+	 * mask for the entire HCD, so don't do that.
+	 */
 	dev->dev.dma_mask = bus->controller->dma_mask;
+	dev->dev.dma_pfn_offset = bus->controller->dma_pfn_offset;
 	set_dev_node(&dev->dev, dev_to_node(bus->controller));
 	dev->state = USB_STATE_ATTACHED;
 	dev->lpm_disable_count = 1;
-- 
2.7.4