linux-kernel - Re: [PATCH V2 4/5] drm/vkms: Use index instead of 0 in possible crtc

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Date:   Tue, 18 Jun 2019 07:56:23 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
Cc:     Brian Starkey <brian.starkey@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        Haneen Mohammed <hamohammed.sa@...il.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 4/5] drm/vkms: Use index instead of 0 in possible crtc

Interestingly, even with the previous code, possible_crtcs=1 was
exposed to userspace [1]. I think this is because of a safeguard in
drm_crtc_init_with_planes (drm_crtc.c:284) which sets the primary and
cursor plane's possible_crtcs to the first CRTC if zero.

If we want to warn on possible_crtcs=0, we should probably remove this
safeguard. Checking first whether this safeguard is used by any driver
is probably a good idea.

[1]: https://drmdb.emersion.fr/devices/f218d1242714