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

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Date:   Fri, 12 Jul 2019 11:30:01 +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

12.07.2019 11:10, Maxime Ripard пишет:
> On Thu, Jul 11, 2019 at 06:55:03PM +0300, Dmitry Osipenko wrote:
>> 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()
> 
> Well, one could argue that video=tegrafb is invalid and should be
> rejected as well, but we haven't cleared that up.

The video=tegrafb is invalid mode, there is nothing to argue here. And
the problem is that invalid modes and not rejected for the very beginning.

>>> 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.
> 
> So we would have the same issue with rotate=0 then?

No, we won't. Rotation mode is parsed into the DRM_MODE bitmask and
rotate=0 corresponds to DRM_MODE_ROTATE_0, which is BIT(0) as you may
notice. Hence rotation_reflection=0 is always an invalid value, meaning
that "rotate" option does not present in the cmdline. Please consult the
code, in particular see drm_mode_parse_cmdline_options() which was
written by yourself ;)