On 26 sep 2009, at 06:54, Alan Stern wrote:
When reading data from a high speed device with usbserial.ko, I
sometimes
get corruct data.
Have you tried using your device on a different computer with a
different sort of motherboard? If the problem still occurs then most
likely it's in the device, not in the computer hardware.
My device is running home-made firmware so it can not be trusted.
However, I'm quite sure that something bad happens in the computer.
I modified usb-serial to fill the transfer buffer with 0xFF just before
submitting the URB. This shows that sometimes the URB returns with
status=0
and the expected value in actual_length, but the last 4 bytes with
respect
to actual_length still contain 0xFF. So clearly the host controller is
screwing up.
The issue is related to alignment of the transfer_buffer address.
The problem does not occur when transfer_buffer is at least 64-byte
aligned.
usb-serial allocates its transfer buffers with kmalloc(). Normally this
would result in aligned buffers. But I was running with
CONFIG_DEBUG_SLAB=y
which changes the alignment guarantees for kmalloc(). After turning off
CONFIG_DEBUG_SLAB, the problem disappeared.
So it seems that my VT62xx EHCI controller will occasionally screw up
a DMA transfer if the transfer buffer is not favorably aligned. Not sure
what to do with that information.
I must say I am also surprised at the behaviour of kmalloc. I'd think
there
are several drivers that implicitly expect aligned buffers from kmalloc.
Greetings, Joris.
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