[PATCH v3] usb: core: setup dma_pfn_offset for USB devices and, interfaces

From: Roger Quadros
Date: Mon Sep 12 2016 - 10:14:32 EST

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
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
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>

- removed comments from code as commit log is sufficient.
- added more information in commit log and code.

drivers/usb/core/message.c | 1 +
drivers/usb/core/usb.c | 1 +
2 files changed, 2 insertions(+)

diff --git a/drivers/usb/core/message.c b/drivers/usb/core/message.c
index 0406a59..6856b7a 100644
--- a/drivers/usb/core/message.c
+++ b/drivers/usb/core/message.c
@@ -1863,6 +1863,7 @@ free_interfaces:
intf->dev.type = &usb_if_device_type;
intf->dev.groups = usb_interface_groups;
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;
diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
index 5e80697..d81791a 100644
--- a/drivers/usb/core/usb.c
+++ b/drivers/usb/core/usb.c
@@ -441,6 +441,7 @@ struct usb_device *usb_alloc_dev(struct usb_device *parent,
dev->dev.type = &usb_device_type;
dev->dev.groups = usb_device_groups;
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;