linux-kernel - Re: [PATCH 0/3] usb: gadget: uvc: allocate requests based on frame interval length and buffersize

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Date: Wed, 22 May 2024 10:50:31 -0400
From: Alan Stern <stern@...land.harvard.edu>
To: Thinh Nguyen <Thinh.Nguyen@...opsys.com>
Cc: Michael Grzeschik <mgr@...gutronix.de>,
  Avichal Rakesh <arakesh@...gle.com>,
  Laurent Pinchart <laurent.pinchart@...asonboard.com>,
  Daniel Scally <dan.scally@...asonboard.com>,
  Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
  Jayant Chowdhary <jchowdhary@...gle.com>,
  "etalvala@...gle.com" <etalvala@...gle.com>,
  Michael Riesch <michael.riesch@...fvision.net>,
  "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
  "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] usb: gadget: uvc: allocate requests based on frame
 interval length and buffersize

On Wed, May 22, 2024 at 01:41:42AM +0000, Thinh Nguyen wrote:
> On Wed, May 22, 2024, Michael Grzeschik wrote:
> > On Fri, May 17, 2024 at 01:44:05AM +0000, Thinh Nguyen wrote:
> > > For isoc endpoint IN, yes. If the host requests for isoc data IN while
> > > no TRB is prepared, then the controller will automatically send 0-length
> > > packet respond.
> > 
> > Perfect! This will help a lot and will make active queueing of own
> > zero-length requests run unnecessary.
> 
> Yes, if we rely on the current start/stop isoc transfer scheme for UVC,
> then this will work.

You shouldn't rely on this behavior.  Other device controllers might not 
behave this way; they might send no packet at all to the host (causing a 
USB protocol error) instead of sending a zero-length packet.

On the other hand, it may not make any difference.  The host's UVC 
driver most likely won't care about the difference between no packet and 
a 0-length packet.  :-)

Alan Stern