linux-kernel - Re: [PATCH v1] drm/modes: Skip invalid cmdline mode

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Message-ID: <20190711090327.keuxt2ztfqecdbef@flea>
Date:   Thu, 11 Jul 2019 11:03:27 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <sean@...rly.run>, Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] drm/modes: Skip invalid cmdline mode

On Wed, Jul 10, 2019 at 06:05:18PM +0300, Dmitry Osipenko wrote:
> 10.07.2019 17:05, Maxime Ripard пишет:
> > On Wed, Jul 10, 2019 at 04:29:19PM +0300, Dmitry Osipenko wrote:
> >> This works:
> >>
> >> diff --git a/drivers/gpu/drm/drm_client_modeset.c b/drivers/gpu/drm/drm_client_modeset.c
> >> index 56d36779d213..e5a2f9c8f404 100644
> >> --- a/drivers/gpu/drm/drm_client_modeset.c
> >> +++ b/drivers/gpu/drm/drm_client_modeset.c
> >> @@ -182,6 +182,8 @@ drm_connector_pick_cmdline_mode(struct drm_connector *connector)
> >>         mode = drm_mode_create_from_cmdline_mode(connector->dev, cmdline_mode);
> >>         if (mode)
> >>                 list_add(&mode->head, &connector->modes);
> >> +       else
> >> +               cmdline_mode->specified = false;
> >
> > Hmmm, it's not clear to me why that wouldn't be the case.
> >
> > If we come back to the beginning of that function, we retrieve the
> > cmdline_mode buffer from the connector pointer, that will probably
> > have been parsed a first time using drm_mode_create_from_cmdline_mode
> > in drm_helper_probe_add_cmdline_mode.
> >
> > Now, I'm guessing that the issue is that in
> > drm_mode_parse_command_line_for_connector, if we have a named mode, we
> > just copy the mode over and set mode->specified.
> >
> > And we then move over to do other checks, and that's probably what
> > fails and returns, but our drm_cmdline_mode will have been modified.
> >
> > I'm not entirely sure how to deal with that though.
> >
> > I guess we could allocate a drm_cmdline_mode structure on the stack,
> > fill that, and if successful copy over its content to the one in
> > drm_connector. That would allow us to only change the content on
> > success, which is what I would expect from such a function?
> >
> > How does that sound?
>
> I now see that there is DRM_MODE_TYPE_USERDEF flag that is assigned only
> for the "cmdline" mode and drm_client_rotation() is the only place in
> DRM code that cares about whether mode is from cmdline, hence looks like
> it will be more correct to do the following:

I'm still under the impression that we're dealing with workarounds of
a more central issue, which is that we shouldn't return a partially
modified drm_cmdline_mode.

You said it yourself, the breakage is in the commit changing the
command line parsing logic, while you're fixing here some code that
was introduced later on.

Can you try the followintg patch?
http://code.bulix.org/8cwk4c-794565?raw

Thanks!
Maxime

--
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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