linux-kernel - Re: dvb usb issues since kernel 4.9

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Date:   Mon, 8 Jan 2018 14:15:35 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Ingo Molnar <mingo@...nel.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        Josef Griebichler <griebichler.josef@....at>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        USB list <linux-usb@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...hat.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        LMML <linux-media@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Miller <davem@...emloft.net>
Subject: Re: dvb usb issues since kernel 4.9

On Mon, 8 Jan 2018, Linus Torvalds wrote:

> Can somebody tell which softirq it is that dvb/usb cares about?

I don't know about the DVB part.  The USB part is a little difficult to
analyze, mostly because the bug reports I've seen are mostly from
people running non-vanilla kernels.  For example, Josef is using a
Raspberry Pi 3B with a non-standard USB host controller driver:
dwc_otg_hcd is built into raspbian in place of the normal dwc2_hsotg
driver.

Both dwc2_hsotg and ehci-hcd use the tasklets embedded in the 
giveback_urb_bh member of struct usb_hcd.  See usb_hcd_giveback_urb() 
in drivers/usb/core/hcd.c; the calls are

        else if (high_prio_bh)
                tasklet_hi_schedule(&bh->bh);
        else
                tasklet_schedule(&bh->bh);

As it turns out, high_prio_bh gets set for interrupt and isochronous
URBs but not for bulk and control URBs.  The DVB driver in question
uses bulk transfers.

xhci-hcd, on the other hand, does not use these tasklets (it doesn't
set the HCD_BH bit in the hc_driver's .flags member).

Alan Stern