From gniibe at fsij.org  Wed Sep  4 02:08:39 2019
From: gniibe at fsij.org (NIIBE Yutaka)
Date: Wed, 04 Sep 2019 09:08:39 +0900
Subject: USB control transfers vs ZLP
In-Reply-To: <alpine.BSO.2.21.1909031323160.24493@resin.csoft.net>
References: <alpine.BSO.2.21.1808142314030.54718@resin.csoft.net>
 <87zhxn82zw.fsf@iwagami.gniibe.org>
 <alpine.BSO.2.21.1808262304190.54718@resin.csoft.net>
 <87va7wqokv.fsf@fsij.org>
 <alpine.BSO.2.21.1909031323160.24493@resin.csoft.net>
Message-ID: <87o9003ou0.fsf@iwagami.gniibe.org>

Jeremy Drake <jeremy at drastrom.science> writes:
> Well, a year later, and I seem to have hit another similar case.  I have a
> HID report descriptor which just happens to be exactly 64 bytes long.  It
> seems like your fix did not properly handle the case when only one packet
> would be needed.

Sorry for my incomplete fix in last summer.  It was too hot.  Hotter
than this year.

I think that I pushed the correct fix finally (over the change of last
year), like:

-  if (data_p->len < USB_MAX_PACKET_SIZE)
+  if (data_p->len <= USB_MAX_PACKET_SIZE)
     {
       len = data_p->len;
       dev->state = LAST_IN_DATA;

> Alas, this does not seem to work with a Windows 10 host.  I have poked it
> with gdb over SWD, and while with Linux len_asked is the correct size (64
> in this case), with Windows len_asked is 64 MORE than the size of the
> report descriptor (128 in this case, but I tested a different descriptor
> which was 56 bytes long and len_asked was 120).  It would seem a ZLP
> should be required in that case, but Windows does not like it, and works
> properly if I disable the ZLP.  Times like these make you wish you could
> take a look at the driver's source and see what they're trying to
> accomplish ;)

For 64-byte descriptor, I believe a single ZLP is required.  Two ZLPs
was sent before the fix.  I guess that it is not good for a Windows 10
host.

For USB full-speed, the packet size is 64-byte.  When len_asked is more
than that for 64-byte descriptor, (I think) a host side expects ZLP
to find end of transaction.

Please test.
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