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Date: Wed, 14 Jan 2009 19:58:44 +0100 From: "Christian Eggers" <ceggers@....de> To: linux-kernel@...r.kernel.org Subject: Buffer allocation for USB transfers 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@....de -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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