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

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Date:   Thu, 11 Jul 2019 18:55:03 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Maxime Ripard <maxime.ripard@...tlin.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

11.07.2019 12:03, Maxime Ripard пишет:
> 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.

The problem stems from assumption that *any* named mode is valid. It
looks to me that the ultimate solution would be to move the mode's name
comparison into the [1], if that's possible.

[1] drm_mode_parse_command_line_for_connector()

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

This doesn't help because the problem with the rotation_reflection is
that it's 0 if "rotation" not present in the cmdline and then ilog2(0)
returns -1. So the patch "drm/modes: Don't apply cmdline's rotation if
it wasn't specified" should be correct in any case.