Re: spec violation of xHCI?

From: Hans Petter Selasky <hps_at_bitfrost.no>
Date: Wed, 11 Dec 2013 15:04:42 +0100
> On 12/11/13 14:06, Kohji Okuno wrote:
>>
>> Hi HPS,
>>
>> All link trbs which are not the end need CHAIN bit, I think.
>> But, this is errata in xHCI ver 0.95. So, linux has quirk for chain
>> bit. Could you check linux codes?
>>
>> Regards,
>>   Kohji Okuno
> 
> Hi Kohji,
> 
> I went through the Linux codes a bit, and I see they have some quirks for the
> chaining bit. Unfortunately Linux does the queuing quite differently than in
> FreeBSD and Shara Sharp which is the author of that code, stated recently a
> need for rewrite of the TRB/TD stuff in Linux, so I'm not sure if that means
> there are more bugs in there or not. Let me explain a bit how things work in
> FreeBSD and why I did not put the chaining bit in line 1895 which you suggest.
> 
> In my design the chaining bit should not be set at line 1895, because if you
> receive a short packet and nframes > 1, the XHCI should not go to the end of
> the frame list, but rather the next frame, nframes + 1.
> 
> If the single short OK flag is set on a BULK transfer, yes, it would be
> correct to set the chaining bit here, but it is not required, because we are
> already are handling activation of the next frame in the function
> "xhci_activate_transfer()" and "xhci_skip_transfer()". Transfer here means
> zero or more TRBs. We use the cycle bit on the TRB to single step the frames
> in software, although you are right that we might optimise this by setting the
> chaining bit instead for the BULK case so that we don't need software
> intervention to handle the job.
> 
> If the multi short OK flag is set, multiple short terminated frames can be
> received and then the chaining bit should not be set.
> 
> Are you seeing a real problem because of the chain bit not being set, or is
> this more the result of code review?
> 
> Thank you for the interest in my XHCI driver.
> 
> --HPS

Hi HPS,

Unfortunately, I can not explain in detail. But, I encountered a real
problem for ZLP. And, when I added CHAIN bit, this problem was
resolved.

When a device driver needs force_short(ZLP), your device driver
creates TRB in the followings.

NORMAL_TRB    - LINK_TRB - NORMAL_TRB - LINK_TRB   => Kick DOORBELL
(with payload)   (#1)      (ZLP)        (#2)

In this case, I think LINK_TRB #1 needs chain bit.

Regards,
 Kohji Okuno