linux-kernel - Re: Allocating buffers for USB transfers (again)

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Date:	Thu, 11 Aug 2011 10:36:47 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andiry Xu <andiry.xu@....com>
cc:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Daniel Mack <zonque@...il.com>,
	Florian Mickler <florian@...kler.org>,
	Oliver Neukum <oliver@...kum.org>, <linux-usb@...r.kernel.org>,
	<alsa-devel@...a-project.org>, Takashi Iwai <tiwai@...e.de>,
	Clemens Ladisch <clemens@...isch.de>, <pedrib@...il.com>,
	William Light <wrl@...est.net>, Greg KH <greg@...ah.com>,
	<linux-kernel@...r.kernel.org>,
	Robert Hancock <hancockrwd@...il.com>
Subject: Re: Allocating buffers for USB transfers (again)

On Thu, 11 Aug 2011, Andiry Xu wrote:

> > Please don't submit zero-length transfers.  The xHCI driver just isn't
> > able to handle it.  Arguably, it probably should have just rejected your
> > URB when it found a zero length buffer, so I'll probably be submitting a
> > patch to fix that.
> > 
> 
> I think queue a zero-length TRB to xhci host is OK. I've not tested it,
> but the issue here seems is caused by td->last_trb = NULL. Check
> count_isoc_trbs_needed(), num_trbs will be 0 if the packet length is
> zero and (addr & (TRB_MAS_BUFF_SIZE - 1)) is zero. We can not return
> num_trbs as 0 to xhci_queue_isoc_tx(), which caused a td added to ep's
> td list, while it's not actually queued to ep ring and last_trb is not
> set.
> 
> In order to avoid this, we just make sure count_isoc_trbs_needed()
> always return 1 or larger numbers, instead of reject the urb. Is that
> feasible?

I just looked for the first time at the code in 
count_isoc_trbs_needed().  It does seem rather sub-optimal.

The basic idea is that you need to know how many TRB buffers can cover 
the memory area spanned by the packet data, where a TRB buffer's size 
cannot be larger than TRB_MAX_BUFF_SIZE and all but the first buffer 
must be aligned on a TRB_MAX_BUFF_SIZE boundary, right?  With a slight 
adjustment in the case of a zero-length packet, since you always need 
at least one TRB.

Given that the packet data starts at addr and has length td_len, the 
number of TRBs should be calculated as follows:

	num = DIV_ROUND_UP(td_len + (addr & (TRB_MAX_BUFF_SIZE - 1))), 
			TRB_MAX_BUFF_SIZE);
	num += (num == 0);

No need for a running_total or a loop.

Alan Stern

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