Re: [PATCH] Bluetooth: virtio_bt: fix device removal

Marcel Holtmann
2021-11-25T21:03:34Z

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.
>>
>
> If you want to do that then device has to be reset on close,
> and fully reinitialized on open.
> Can you work on a patch like that?
> Given I don't have the device such a rework is probably more
> than I can undertake.

so you mean move virtio_find_vqs() into virtbt_open() and del_vqs() into
virtbt_close()?

Or is there are way to set up the queues without starting them?

However I am failing to understand your initial concern, we do reset()
before del_vqs() in virtbt_remove(). Should we be doing something different
in virtbt_close() other than virtqueue_detach_unused_buf(). Why would I
keep buffers attached if they are not used.

Regards

Marcel