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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@ti.com> Acked-by: Arnd Bergmann <arnd@arndb.de> Acked-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To understand all the Linux-USB framework, you'll use these resources:
* This source code. This is necessarily an evolving work, and
includes kerneldoc that should help you get a current overview.
("make pdfdocs", and then look at "usb.pdf" for host side and
"gadget.pdf" for peripheral side.) Also, Documentation/usb has
more information.
* The USB 2.0 specification (from www.usb.org), with supplements
such as those for USB OTG and the various device classes.
The USB specification has a good overview chapter, and USB
peripherals conform to the widely known "Chapter 9".
* Chip specifications for USB controllers. Examples include
host controllers (on PCs, servers, and more); peripheral
controllers (in devices with Linux firmware, like printers or
cell phones); and hard-wired peripherals like Ethernet adapters.
* Specifications for other protocols implemented by USB peripheral
functions. Some are vendor-specific; others are vendor-neutral
but just standardized outside of the www.usb.org team.
Here is a list of what each subdirectory here is, and what is contained in
them.
core/ - This is for the core USB host code, including the
usbfs files and the hub class driver ("hub_wq").
host/ - This is for USB host controller drivers. This
includes UHCI, OHCI, EHCI, and others that might
be used with more specialized "embedded" systems.
gadget/ - This is for USB peripheral controller drivers and
the various gadget drivers which talk to them.
Individual USB driver directories. A new driver should be added to the
first subdirectory in the list below that it fits into.
image/ - This is for still image drivers, like scanners or
digital cameras.
../input/ - This is for any driver that uses the input subsystem,
like keyboard, mice, touchscreens, tablets, etc.
../media/ - This is for multimedia drivers, like video cameras,
radios, and any other drivers that talk to the v4l
subsystem.
../net/ - This is for network drivers.
serial/ - This is for USB to serial drivers.
storage/ - This is for USB mass-storage drivers.
class/ - This is for all USB device drivers that do not fit
into any of the above categories, and work for a range
of USB Class specified devices.
misc/ - This is for all USB device drivers that do not fit
into any of the above categories.