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Date:   Fri, 16 Feb 2018 21:38:08 +0100
From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Mauro Carvalho Chehab <mchehab@s-opensource.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, linux-usb@vger.kernel.org
Subject: Re: [RFC PATCH] usb: hcd: complete URBs in threaded-IRQ context
 instead of tasklet
Message-ID: <20180216203808.23mvv4mtz6khhwfv@linutronix.de>
References: <20180216170450.yl5owfphuvltstnt@breakpoint.cc>
 <Pine.LNX.4.44L0.1802161325050.1402-100000@iolanthe.rowland.org>
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On 2018-02-16 13:29:01 [-0500], Alan Stern wrote:
> We originally used tasklets because we didn't want to incur the delays 
> associated with running in a process context.  It seems odd to be 
> reversing that decision now.

The theaded interrupt runs SCHED_FIFO priority 50 by default. The only
thing that can interrupt it are interrupts, a softirq (not ksoftirqd)
and other tasks with a higher priority than 50.
There should be no downside performance wise.

> > The URBs from the root-hub never create an interrupt so I currently
> > process them in a workqueue (I'm not sure if an URB-enqueue in the
> > completion handler would break something).
> 
> It worked okay before we changed over to using tasklets.

Ah okay. I've seen that HCDs were no longer dropping their internal lock
and I wasn't sure if such a change was also applied in RH-code.

> Alan Stern

Sebastian