Re: spec violation of xHCI?

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