linux-kernel - Re: [PATCH 4.19 19/57] Bluetooth: hidp: Let hidp_send

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Date:   Mon, 9 Sep 2019 23:59:29 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Fabian Henneke <fabian.henneke@...il.com>
Cc:     Pavel Machek <pavel@...x.de>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Marcel Holtmann <marcel@...tmann.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 19/57] Bluetooth: hidp: Let hidp_send_message return
 number of queued bytes

On Mon, Sep 09, 2019 at 03:00:46PM +0200, Fabian Henneke wrote:
> Hi,
> 
> On Mon, Sep 9, 2019 at 2:15 PM Pavel Machek <pavel@...x.de> wrote:
> 
> > Hi!
> >
> > > [ Upstream commit 48d9cc9d85dde37c87abb7ac9bbec6598ba44b56 ]
> > >
> > > Let hidp_send_message return the number of successfully queued bytes
> > > instead of an unconditional 0.
> > >
> > > With the return value fixed to 0, other drivers relying on hidp, such as
> > > hidraw, can not return meaningful values from their respective
> > > implementations of write(). In particular, with the current behavior, a
> > > hidraw device's write() will have different return values depending on
> > > whether the device is connected via USB or Bluetooth, which makes it
> > > harder to abstract away the transport layer.
> >
> > So, does this change any actual behaviour?
> >
> > Is it fixing a bug, or is it just preparation for a patch that is not
> > going to make it to stable?
> >
> 
> I created this patch specifically in order to ensure that user space
> applications can use HID devices with hidraw without needing to care about
> whether the transport is USB or Bluetooth. Without the patch, every
> hidraw-backed Bluetooth device needs to be treated specially as its write()
> violates the usual return value contract, which could be viewed as a bug.
> 
> Please note that a later patch (
> https://www.spinics.net/lists/linux-input/msg63291.html) fixes some
> important error checks that were relying on the old behavior (and were
> unfortunately missed by me).

As that patch doesn't seem to be in Linus's tree yet, we should postpone
taking this one in the stable tree right now, correct?

thanks,

greg k-h