linux-kernel - [PATCH] usb: core: setup dma_pfn

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Date:	Wed, 17 Aug 2016 15:35:28 +0300
From:	Roger Quadros <rogerq@...com>
To:	<gregkh@...uxfoundation.org>, <stern@...land.harvard.edu>
CC:	<ssantosh@...nel.org>, <grygorii.strashko@...com>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Roger Quadros <rogerq@...com>
Subject: [PATCH] usb: core: setup dma_pfn_offset for USB devices

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.

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@...com>
---
 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;
 		device_initialize(&intf->dev);
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;
-- 
2.7.4