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Date:   Tue, 28 Jun 2022 00:59:10 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Marcel Holtmann <marcel@holtmann.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        linux-bluetooth@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Jason Wang <jasowang@redhat.com>
Subject: Re: [PATCH] Bluetooth: virtio_bt: fix device removal
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On Mon, Jun 13, 2022 at 02:58:59AM -0400, Michael S. Tsirkin wrote:
> On Fri, Jan 14, 2022 at 03:12:47PM -0500, Michael S. Tsirkin wrote:
> > On Thu, Dec 16, 2021 at 08:58:31PM +0100, Marcel Holtmann wrote:
> > > Hi Michael,
> > > 
> > > >>>>> Device removal is clearly out of virtio spec: it attempts to remove
> > > >>>>> unused buffers from a VQ before invoking device reset. To fix, make
> > > >>>>> open/close NOPs and do all cleanup/setup in probe/remove.
> > > >>>> 
> > > >>>> so the virtbt_{open,close} as NOP is not really what a driver is suppose
> > > >>>> to be doing. These are transport enable/disable callbacks from the BT
> > > >>>> Core towards the driver. It maps to a device being enabled/disabled by
> > > >>>> something like bluetoothd for example. So if disabled, I expect that no
> > > >>>> resources/queues are in use.
> > > >>>> 
> > > >>>> Maybe I misunderstand the virtio spec in that regard, but I would like
> > > >>>> to keep this fundamental concept of a Bluetooth driver. It does work
> > > >>>> with all other transports like USB, SDIO, UART etc.
> > > >>>> 
> > > >>>>> The cost here is a single skb wasted on an unused bt device - which
> > > >>>>> seems modest.
> > > >>>> 
> > > >>>> There should be no buffer used if the device is powered off. We also don’t
> > > >>>> have any USB URBs in-flight if the transport is not active.
> > > >>>> 
> > > >>>>> NB: with this fix in place driver still suffers from a race condition if
> > > >>>>> an interrupt triggers while device is being reset. Work on a fix for
> > > >>>>> that issue is in progress.
> > > >>>> 
> > > >>>> In the virtbt_close() callback we should deactivate all interrupts.
> > > >>>> 
> > > >>>> Regards
> > > >>>> 
> > > >>>> Marcel
> > > >>> 
> > > >>> So Marcel, do I read it right that you are working on a fix
> > > >>> and I can drop this patch for now?
> > > >> 
> > > >> ping
> > > > 
> > > > 
> > > > If I don't hear otherwise I'll queue my version - it might not
> > > > be ideal but it at least does not violate the spec.
> > > > We can work on not allocating/freeing buffers later
> > > > as appropriate.
> > > 
> > > I have a patch, but it is not fully tested yet.
> > > 
> > > Regards
> > > 
> > > Marcel
> > 
> > ping
> > 
> > it's been a month ...
> > 
> > I'm working on cleaning up module/device removal in virtio and bt
> > is kind of sticking out.
> 
> I am inclined to make this driver depend on BROKEN for now.
> Any objections?

OK patch incoming.

> 
> > -- 
> > MST