Kernel v6.1: Plane IDs values incremented by 2, why ?

[Laurent Pinchart]

Hi Sunil,

On Thu, Jan 19, 2023 at 12:08:42PM +0000, Vaghela, Sunil wrote:
> [AMD Official Use Only - General]

This kind of header may deter some people from replying on mailing
lists. You may want to check if there's an outgoing mail server that you
can use for public discussions that wouldn't add this.

> Hi,
> 
> With Kernel v5.15 -> v6.1 upgradation, I have noticed that, now consecutive
> Plane IDs values are incremented by 2 each time. i.e. 34,36,38. In kernel
> v5.15, it was incremented by 1. i.e. 34,35,36…
> 
> I attached modetest log with kernel v6.1.
> 
> Is this change expected with new Kernel v6.1 ? If Yes, can you please elaborate
> the requirement/change.

As far as I understand, DRM object IDs are not part of the DRM ABI and
can thus change at any time, between releases or even between boots.
Userspace should not rely on any object having a specific ID.

As for why the IDs are not contiguous anymore, I believe that because of
the blob create by the call to create_in_format_blob() at the end of the
__drm_universal_plane_init() function. Blobs are DRM objects, so they're
assigned an ID.

-- 
Regards,

Laurent Pinchart