Re: usb_control_msg() reads 0 bytes in Linux 2.6.23-rc1

On Tue, 24 Jul 2007 16:28:15 -0400, Pavel Roskin <[email protected]> wrote:

> It turns out usb_control_msg() returns 0 in two cases where it returns
> -EPIPE in Linux 2.6.22.

This is significant. EPIPE is a "stall", but zero bytes is zero bytes.
Normally they do not mix. If a device stalls a request, it's different
from returning no bytes of data. Actually, I'm wondering if your tests
were clean enough... If you load firmware, a device can alter its behaviour
drastically, and switch from stalls to empty replies or normal replies
or whatever.

> Return value 0 is not expected to be an error,
> which prevents second stage of the firmware download, normally triggered
> by an error reading the firmware version.

Well, as you are aware now, it's a separate condition which you have
to check. A device can return zero bytes, or one byte, or whatever.
If it's not supposed to do that, you have to take some corrective action,
like a reset.

> However, I think it's wrong to return 0 if more than 0 bytes were
> requested.  usb_control_msg() is synchronous, and the caller is supposed
> to wait for the data.  If the function returns and the data is not
> there, it should be an error IMHO.

I disagree, and in any case it's too late to do anything about it.

BTW, read(2) can return short reads in many circumstances where
you opened a character device without O_NONBLOCK. Consider a synchronous
link. You issue a read for maximum packet size, and then receive the
actual number of bytes, or even a zero.

Please keep in mind that all this discussion about short reads has
nothing to do with your real problem (e.g. the device which used to
reply with a stall stopped to do so, or it continues to reply with a
stall token, but EHCI fails to report the stall up the stack...)

-- Pete

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