[Intel-gfx] [PATCH] drm/crtc: constify drm_crtc_index parameter

[Daniel Vetter]

On Mon, Oct 10, 2016 at 6:04 PM, Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
> On Mon, Oct 10, 2016 at 06:26:10PM +0300, Jani Nikula wrote:
>> Signed-off-by: Jani Nikula <jani.nikula at intel.com>
>>
>> ---
>>
>> I needed this for some stuff that turned out to be a dead end. But this
>> seems to be the right thing to do anyway.
>> ---
>>  include/drm/drm_crtc.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
>> index 61932f55f788..0aa292526567 100644
>> --- a/include/drm/drm_crtc.h
>> +++ b/include/drm/drm_crtc.h
>> @@ -1342,7 +1342,7 @@ extern void drm_crtc_cleanup(struct drm_crtc *crtc);
>>   * Given a registered CRTC, return the index of that CRTC within a DRM
>>   * device's list of CRTCs.
>>   */
>> -static inline unsigned int drm_crtc_index(struct drm_crtc *crtc)
>> +static inline unsigned int drm_crtc_index(const struct drm_crtc *crtc)
>>  {
>>       return crtc->index;
>
> BTW speaking about the index stuff. It dawned on me recently that calling
> drm_crtc_cleanup() & co. is totally not safe except maybe during the
> final cleanup.
>
> If you would do something like:
> a = drm_crtc_init();
> b = drm_crtc_init();
> drm_crtc_cleanup(a);
> c = drm_crtc_init();
>
> b and c would end up with the same index.
>
>
> Or if you would do just
> a = drm_crtc_init();
> b = drm_crtc_init();
> drm_crtc_cleanup(a);
>
> We'd end up with num_crtc==1, but b->index==1, so we'd actually access
> beyond the allocated arrays in a bunch of places.
>
> This would need to fixed somehow, or at least documented clearly that if
> you have to call any of the cleanup functions during init, you have to
> abort the entire thing.

You can't hotplug CRTCs, like you can't hotplug anything else than
connectors. And for connectors we have an ida to make sure you never
end up with duplicated indizes. So I think we're safe.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch