[PATCH] drm: add check for plane functions

Harry Wentland harry.wentland at amd.com
Tue Mar 21 15:27:49 UTC 2017

On 2017-03-20 05:42 AM, Shirish S wrote:
> On Mon, Mar 20, 2017 at 1:51 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
>> On Mon, Mar 20, 2017 at 09:58:01AM +0530, Shirish S wrote:
>>> First of all, thanks for your comments/insights.
>>> On Sat, Mar 18, 2017 at 12:59 AM, Eric Anholt <eric at anholt.net> wrote:
>>>> Ville Syrjälä <ville.syrjala at linux.intel.com> writes:
>>>>> On Fri, Mar 17, 2017 at 05:57:52PM +0100, Daniel Vetter wrote:
>>>>>> On Fri, Mar 17, 2017 at 01:08:43PM +0200, Ville Syrjälä wrote:
>>>>>>> On Fri, Mar 17, 2017 at 03:46:34PM +0530, Shirish S wrote:
>>>>>>>> On Fri, Mar 17, 2017 at 3:26 PM, Ville Syrjälä
>>>>>>>> <ville.syrjala at linux.intel.com> wrote:
>>>>>>>>> On Fri, Mar 17, 2017 at 01:25:08PM +0530, Shirish S wrote:
>>>>>>>>>> update_plane() and disable_plane() functions
>>>>>>>>>> assoiciated with setting plane are called
>>>>>>>>>> without any check, causing kernel panic.
>>>>>>>>> Why are you registering a plane without the funcs?
>>>>>>>> Basically, enabling planes and making them fully functional is
>>>>>>>> generally a 2 -step process,
>>>>>>>> so i suggest for new drivers wanting to implement/re-design  planes,
>>>>>>>> would like to tap
>>>>>>>> the flow at enabling(listing caps) and later at ensuring it works.
>>>>>>> I don't think there's much point in exposing something that
>>>>>>> doesn't work. And even if you do, you could always just use
>>>>>>> stub functions.
>>>>>> Yes, just wire up stub functions if you want to enable planes with
>>>>>> multi-step patch series.
>>>>>>>> I noticed that there is a underlying assumption only for
>>>>>>>> plane->(funcs) are implemented, whereas for
>>>>>>>> other function for crtc/connector/encoder function calls there is a
>>>>>>>> sanity check(or WARN_ON) through out the framework.
>>>>>>>> I believe this check wont cause any performance/functional impact.
>>>>>>>> Please let me know if am missing anything.
>>>>>>>> And further more help developers to focus on enabling planes via
>>>>>>>> various tests without causing reboots/system hangs.
>>>>>>> I don't particularly like adding more unconditional runtime checks
>>>>>>> that just to protect developers from themselves. If you really
>>>>>>> think there's value in these, then at least add the checks into
>>>>>>> the plane init codepath so that it's a one time cost.
>>> All the plane->funcs are guarded before being called , be it:
>>>              late_register()
>>>              early_unregister()
>>>             atomic_destroy_state() etc.,
>>> only update/disable_plane() are called without checking their
>>> existence, am just extending  the protocol.
>>>>>>> The same approach could be used for all the other non-optional
>>>>>>> hooks. Otherwise the same WARN_ON()s would have to be sprinkled
>>>>>>> all over the place, and there's always the risk of missing a few
>>>>>>> codepaths that call a specific hook.
>>>>>> I think for these here there's negative value - it allows developers to
>>>>>> create completely broken planes. Stub functions really seem like a much
>>>>>> better idea.
>>>>> I was thinking
>>>>> drm_whatever_init()
>>>>> {
>>>>>       if (WARN_ON(!funcs->mandatory_thing))
>>>>>               return -EINVAL;
>>>>> }
>>> I think since the motive here is to
>>> * convey user space that it does not have permissions to
>>> update/disable available plane due to implementation issues.
>>> * Keeping system alive/usable after non-permitted call.
>>> Adding  a WARN_ON() trace showing something is missing at boot/insmod
>>> time, wont solve the purpose.
>>> This  development phase here could be setting-up infra for adding a
>>> plane available on hardware,populate its capabilities
>>> and to know how user space reads it and tweak it before moving to
>>> configuring registers.
>>> To add to what @Eric Anholt mentioned, without this patch developer
>>> comes to know about
>>> the mandatory functions required in a real tough way of panic and
>>> system freezes,
>>> just because the core framework invokes a NULL function pointer
>>> without checking.
>>> (Am re-stressing here, that only update/disable planes are exceptions
>>> rest all have required checks.)
>> Eric acked Ville's idea, not your patch.
>>>>> rather than putting the WARN_ON()s around each call of
>>>>> funcs->mandatory_thing().
>>> There are similar checks around every
>>> "[crtc/encoder]->funcs->[hooked_up_function specific to vendor]",
>>> including  plane functions called in drm_plane.c & other places like:
>>>      drivers/gpu/drm/drm_crtc_helper.c:1074: if
>>> (plane->funcs->atomic_duplicate_state)
>>>      drivers/gpu/drm/drm_mode_config.c:176:          if (plane->funcs->reset)
>>>      drivers/gpu/drm/drm_plane.c:162:                if
>>> (plane->funcs->late_register)
>>>      drivers/gpu/drm/drm_plane.c:242:        if (plane->state &&
>>> plane->funcs->atomic_destroy_state)
>>> and so on...
>>> For consistency sake lets have this check.
>> Those are different functions. They are in transitional helpers, where
>> we explicitly assume not all the atomic bits are ready yet.
>> Different use-case, different semantics.
>>>>> That will fail gracefully (which I guess is what people are after here),
>>>>> and gives the developer a clear message what's missing.
>>>> Having this in our init functions for funcs and helpers would have saved
>>>> me tons of time in vc4 and clcd.
>>>> _______________________________________________
>>>> dri-devel mailing list
>>>> dri-devel at lists.freedesktop.org
>>>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>> Thanks again for your comments, all am trying here is to only fix a
>>> bug that shall enable developers in a positive way.
>> See Ville's proposal, I think that's a good idea. Volunteered to review
>> the various docs and make sure we have these checks in the various _init()
>> functions?
>> -Daniel
> As i mentioned earlier also, WARN_ON() wont solve the purpose,
> the panic cant be avoided making the system unusable.
> May be i will use a WIP patch internally till everything is in place.
> Thanks for your inputs.

I was MIA yesterday so I'm a bit late to the discussion. I like Ville's 
idea, though.

Just to rephrase: the only function pointers we want to NULL check at 
runtime are those that are deliberately left to be optional. All 
mandatory functions are allowed to crash at runtime.

Ville's suggestion is to make apparent inside our _init functions which 
function pointers are mandatory.

You can still keep the existing patch for your own benefit but I don't 
think it'll fly for upstream.

I'll leave it up to you whether you want to implement the NULL function 
pointer checks in the _init functions.


>> --
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch
> Regards,
> Shirish S
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