From: "Christian Eggers" <ceggers@gmx.de>
To: linux-kernel@vger.kernel.org
Date: Wed, 14 Jan 2009 19:58:44 +0100
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Subject: Buffer allocation for USB transfers
Message-ID: <496E43F4.24792.158F9B@ceggers.gmx.de>
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In different drivers I've found several methods for allocating buffers 
transfered with usb_control_msg() or usb_submit_urb():

- usb_stor_msg_common() in "drivers/usb/storage/transport.c" uses buffers 
allocated with usb_buffer_alloc(). These buffers are used with 
URB_NO_xxx_DMA_MAP in urb->transfer_flags.

- asix_read_cmd() in "drivers/net/usb/asix.c" uses kmalloc(GFP_KERNEL).

- mcs7830_get_reg() in "drivers/net/usb/mcs7830.c" uses buffers from 
the stack.

At least the latter does not work on my SH-4 platform. It seems that other 
variables on the stack are overwritten after calling usb_control_msg(), probably as 
result of incorrect alignment.

For some reason the second example (kmalloc()) doesn't seem to cause problems (on 
my platform) but is there are guarantee that kmalloc() 
without GFP_DMA does always return a DMA capable buffer?

Shall all buffers used for usb_control_msg() and usb_submit_urb() be 
allocated with usb_buffer_alloc()? It seems that usb_control_msg() doesn't 
offer a way to set the URB_NO_xxx_DMA_MAP in urb->transfer_flags so that 
usb_buffer_alloc() can not be used here???

regards
Christian Eggers

Please CC to ceggers@gmx.de

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