linux-kernel - Re: [PATCH 2/3] usb: dwc2: host: Giveback URB in tasklet context

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Date:	Thu, 5 Nov 2015 10:19:00 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Doug Anderson <dianders@...omium.org>
cc:	John Youn <John.Youn@...opsys.com>, Felipe Balbi <balbi@...com>,
	Yunzhi Li <lyz@...k-chips.com>,
	Heiko Stübner <heiko@...ech.de>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	Julius Werner <jwerner@...omium.org>,
	"Herrero, Gregory" <gregory.herrero@...el.com>,
	"Kaukab, Yousaf" <yousaf.kaukab@...el.com>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	John Youn <johnyoun@...opsys.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ming Lei <ming.lei@...onical.com>
Subject: Re: [PATCH 2/3] usb: dwc2: host: Giveback URB in tasklet context

On Wed, 4 Nov 2015, Doug Anderson wrote:

> In the ChromeOS gerrit
> <https://chromium-review.googlesource.com/#/c/310583/> Julius Werner
> points out that for EHCI it was good to take the optimization from
> commit 9118f9eb4f1e ("USB: EHCI: improve interrupt qh unlink") before
> this one.  I'm still trying to learn USB / dwc2 so it's unclear to me
> whether we also need a similar change before landing.
> 
> I'll see if I can do some investigation about this and also some
> benchmarking before and after.  Certainly profiling the interrupt
> handler itself showed a huge improvement, but I'd hate to see a
> regression elsewhere.
> 
> If anyone else knows better than I, please speak up!  :)

This is a matter of both efficiency and correctness.  Giving back URBs 
in a tasklet is not a simple change.

Have you read the kerneldoc for usb_submit_urb() in 
drivers/usb/core/urb.c?  The portion about "Reserved Bandwidth 
Transfers" is highly relevant.  I don't know how dwc2 goes about 
reserving bandwidth for periodic transfers, but if it relies on the 
endpoint queue being non-empty to maintain a reservation then it will 
be affected by this change.

Alan Stern

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