On Fri, 3 Jul 2009, Yang Fei-AFY095 wrote:
quoted text > Hello All,
>
> I'm working on a TI OMAP based device running linux kernel 2.6.29. I
> encountered a problem with the USB host driver and would like to get
> some advice on a safe solution.
>
> The problem happens within the following code of function "int
> usb_get_configuration(struct usb_device *dev)" in
> drivers/usb/core/config.c
...
quoted text > bigbuffer is a cacheable buffer holding the device configuration
> descriptor. Sometimes the function usb_parse_configuration complains
> "config 1 has an invalid descriptor of length 0, skipping remainder of
> the config", because the bLength field of that descriptor is 0. This
> error results in enumeration failure for that interface. The issue is
> not 100% reproducible, and the worst occurrence rate I have seen is
> about 50%.
>
> After some debugging, I think the problem is probably caused by
> inconsistency between cache and physical memory, the CPU is reading
> values from cache while the host controller DMA the descriptors directly
> to the physical memory. Based on this analysis, I added
> "dma_cache_maint(bigbuffer, length, DMA_FROM_DEVICE);" before calling
> usb_parse_configuration, and that seems to fix the problem. But this is
> just part of the analysis, answer to the following questions would
> greatly help on finding the right solution.
>
> 1. I see dma_cache_maint is called by usb_submit_urb before starting
> data transfer,
It is? Where?
quoted text > this would guarantee the integrity of OUT data. But for
> IN data, is the cache consistency taken care of by this driver?
For both IN and OUT transfers, cache consistency is handled by
map_urb_for_dma and unmap_urb_for_dma. Those two functions are in
hcd.c.
quoted text > 2. Is it possible that this issue is caused by some other settings in
> the system, such as clock settings, cache configuration or the host
> controller settings? (we are using a EHCI controller)
Possibly.
quoted text > 3. Are we able to disable DMA for all EP0 transactions? As the amount of
> data is relatively less, DMA is not really necessary.
You can disable DMA for ep0 if you want. It shouldn't make much
difference.
But if you're getting these errors already, maybe you'll get similar
errors with transfers to other endpoints. So it might be better to fix
the problem than to try working around it.
Alan Stern
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