linux-kernel - RE: [PATCH] media: v4l2-compat-ioctl32.c: copy reserved2 field in get_v4l2

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Date:   Tue, 21 Apr 2020 12:33:42 +0900
From:   "Sunyoung Kang" <sy0816.kang@...sung.com>
To:     "'Arnd Bergmann'" <arnd@...db.de>
Cc:     "'Greg Kroah-Hartman'" <gregkh@...uxfoundation.org>,
        "'Mauro Carvalho Chehab'" <mchehab@...nel.org>,
        "'Hans Verkuil'" <hverkuil-cisco@...all.nl>,
        "'Thomas Gleixner'" <tglx@...utronix.de>,
        "'Linux Media Mailing List'" <linux-media@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] media: v4l2-compat-ioctl32.c: copy reserved2 field in
 get_v4l2_buffer32

Thank you for your detailed guide.
And I'll look into how to handle the additional information.

Thanks
Sunyoung

> -----Original Message-----
> From: Arnd Bergmann <arnd@...db.de>
> Sent: Monday, April 20, 2020 8:23 PM
> To: Sunyoung Kang <sy0816.kang@...sung.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>; Mauro Carvalho Chehab
> <mchehab@...nel.org>; Hans Verkuil <hverkuil-cisco@...all.nl>; Thomas
> Gleixner <tglx@...utronix.de>; Linux Media Mailing List <linux-
> media@...r.kernel.org>; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] media: v4l2-compat-ioctl32.c: copy reserved2 field in
> get_v4l2_buffer32
> 
> On Mon, Apr 20, 2020 at 2:40 AM Sunyoung Kang <sy0816.kang@...sung.com>
> wrote:
> >
> > I understand what you mean.
> > However, the way to transmit information about the buffer is only
> > flags in v4l2_buffer In flags in v4l2_buffer, there is no reserved bit
> > field that can be used for custom.
> > Additional information about the buffer is needed to provide various
> > functions required by the customers but flags is not enough. So
> > reserved2 is used as an alternative.
> > Can you suggest a better opinion?
> 
> If you have a driver that needs to pass additional information that is not
> supported by the subsystem, this is generally either because there is
> something wrong in the driver, or because there is something wrong in the
> subsystem.
> 
> Whichever is at fault should be fixed. If it's the subsystem, then you
> should explain why it's wrong and make a suggestion for how to address it,
> e.g.
> introducing a new ioctl command or redefining the reserved members to be
> defined in the way you need.
> 
> In any case, the ioctl commands should be driver independent, so that any
> hardware with the same feature as your driver can work with the same user
> space.
> 
>      Arnd