linux-kernel - Re: [RFC 1/4] drm: Add writeback connector type

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Message-ID: <20180223170432.GP223881@art_vandelay>
Date:   Fri, 23 Feb 2018 12:04:32 -0500
From:   Sean Paul <seanpaul@...omium.org>
To:     Liviu Dudau <liviu.dudau@....com>
Cc:     Sean Paul <seanpaul@...omium.org>, Rob Clark <robdclark@...il.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Brian Starkey <brian.starkey@....com>,
        Mihail Atanassov <mihail.atanassov@....com>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        daniel.vetter@...ll.ch
Subject: Re: [RFC 1/4] drm: Add writeback connector type

On Fri, Feb 23, 2018 at 04:48:58PM +0000, Liviu Dudau wrote:
> On Fri, Feb 23, 2018 at 11:43:29AM -0500, Sean Paul wrote:
> > On Fri, Feb 23, 2018 at 11:25:11AM -0500, Rob Clark wrote:
> > > On Fri, Feb 23, 2018 at 10:59 AM, Sean Paul <seanpaul@...omium.org> wrote:
> > > >
> > > > Have we considered hiding writeback behind a client cap instead?
> > > 
> > > It is kinda *almost* unneeded, since the connector reports itself as
> > > disconnected.
> > > 
> > > I'm not sure what the reason was to drop the cap, but I think it would
> > > be better to have a cap so WB connectors don't show up in, for ex,
> > > xrandr
> > 
> > Yeah, the disconnected hack is kind of gross, IMO. I hate to introduce churn in
> > the patch series given that it was initially introduced with the client cap.
> 
> Haha, that's the reverse of Daniel's position:
> 
> https://lists.freedesktop.org/archives/dri-devel/2016-October/120519.html

Yeah, it happens :(. I don't think it's a dealbreaker either way, it just seems
awkward to expose a connector which is "disconnected", but available for use. I
don't think we have any other connectors which are supposed to be used in this
state.

> 
> > 
> > There are also cases where we might want to make writeback unavailable, such as
> > when content protection is enabled. In those cases, it's conceivable that we
> > might want to use disconnected as a signal to u/s. I suppose we could also just
> > fail the check, so most of this is just academic.
> 
> Not sure what other hardware out there does, but on Mali DP's case you
> would be outputing the protected content by putting the display
> processor in secure mode, which automatically disables writeback for us.
> Or to put in another way, you don't need a writeback framebuffer if you
> are in non-secure mode as you can get access to the framebuffer used for
> the plane anyway.

Yeah, I was mostly thinking about the case where you might have HDCP enabled on
the HDMI connector and be able to slurp up the content via a writeback. However
if the buffer is not secure in the first place, then it's already accessible in
userspace so I don't think that writeback presents a new security hole.

/me needs to get HDCP out of his head.

Sean


> 
> Best regards,
> Liviu
> 
> > 
> > Sean
> > 
> > 
> > > 
> > > BR,
> > > -R
> > 
> > -- 
> > Sean Paul, Software Engineer, Google / Chromium OS
> 
> -- 
> ====================
> | I would like to |
> | fix the world,  |
> | but they're not |
> | giving me the   |
>  \ source code!  /
>   ---------------
>     ¯\_(ツ)_/¯

-- 
Sean Paul, Software Engineer, Google / Chromium OS