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SubjectRe: [PATCH 0/2] Reset realtek bluetooth devices during user suspend
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On Wed, Sep 18, 2019 at 7:23 AM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Tue, 17 Sep 2019, Abhishek Pandit-Subedi wrote:
>
> > On a Realtek USB bluetooth device, I wanted a simple and consistent way
> > to put the device in reset during suspend (2 reasons: to save power and
> > disable BT as a wakeup source). Resetting it in the suspend callback
> > causes a detach and the resume callback is not called. Hence the changes
> > in this series to do the reset in suspend_noirq.
>
> What about people who _want_ BT to be a wakeup source?

When BT is enabled as a wakeup source, there is no reset.

> Why does putting the device in reset save power? That is, a suspended
> device is very strictly limited in the amount of current it's allowed
> to draw from the USB bus; why should it draw significantly less when it
> is reset?

I don't know that it's significantly less (only that it's OFF). My
greater motivation is to make sure the bluetooth chip isn't
accumulating events while the host is turned off. Sorry, I should have
made that more clear in the cover letter.

When the host is off, it continues to accumulate events for the host
to process (packets from connected devices, LE advertisements, etc).
At some point, the firmware buffers fill up and no more events can be
stored. When the host is resumed later on, the firmware is in a bad
state and doesn't respond. I had originally just reset in ->resume but
then connected wireless devices wouldn't disconnect from the BT either
and I had trouble getting them to reconnect.

>
> > I looked into using PERSIST and reset on resume but those seem mainly
> > for misbehaving devices that reset themselves.
>
> They are, but that doesn't mean you can't use them for other things
> too.
>
> > This patch series has been tested with Realtek BT hardware as well as
> > Intel BT (test procedure = disable as wake source, user suspend and
> > observe a detach + reattach on resume).
>
> This series really seems like overkill for a single kind of device.
>
> Is there any way to turn off the device's BT radio during suspend (if
> wakeup is disabled) and then turn it back on during resume? Wouldn't
> that accomplish what you want just as well?

Probably (but I couldn't find a way to do that). I want to prevent
bluetooth from waking up the host and to reliably be in a good state
when the host resumes. The reset logic I implemented causes the hci
device to disappear and reappear, which userspace seems to handle
gracefully.

> Alan Stern
>

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