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

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Date:   Mon, 9 Sep 2019 15:00:46 +0200
From:   Fabian Henneke <fabian.henneke@...il.com>
To:     Pavel Machek <pavel@...x.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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

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).

Best regards,
Fabian

>                                                         Pavel
>
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures)
> http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
>