linux-kernel - Re: [PATCH] Bluetooth: virtio

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Date:   Mon, 13 Dec 2021 18:57:35 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Marcel Holtmann <marcel@...tmann.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        linux-bluetooth@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] Bluetooth: virtio_bt: fix device removal

On Mon, Dec 13, 2021 at 05:44:13AM -0500, Michael S. Tsirkin wrote:
> On Thu, Dec 09, 2021 at 04:22:58PM -0500, Michael S. Tsirkin wrote:
> > On Thu, Nov 25, 2021 at 09:02:01PM +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.

> > -- 
> > MST