RE: [RESEND RFC 18/18] drm/display/dp_mst: Move all payload info into the atomic state

[Lin, Wayne]

[Public]



> -----Original Message-----
> From: Lyude Paul <lyude@xxxxxxxxxx>
> Sent: Thursday, August 4, 2022 4:28 AM
> To: Lin, Wayne <Wayne.Lin@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx;
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> Subject: Re: [RESEND RFC 18/18] drm/display/dp_mst: Move all payload info
> into the atomic state
> 
> On Tue, 2022-07-05 at 09:10 +0000, Lin, Wayne wrote:
> > > +struct drm_dp_mst_port;
> > > +
> > >   /* DP MST stream allocation (payload bandwidth number) */
> > >   struct dc_dp_mst_stream_allocation {
> > >    uint8_t vcp_id;
> > >    /* number of slots required for the DP stream in
> > >    * transport packet */
> > >    uint8_t slot_count;
> > > + /* The MST port this is on, this is used to associate DC MST
> > > + payloads
> > > with their
> > > + * respective DRM payloads allocations, and can be ignored on non-
> > > Linux.
> > > + */
> >
> > Is it necessary for adding this new member? Since this is for setting
> > the DC HW and not relating to drm.
> 
> I don't entirely know, honestly. The reasons I did it:
> 
>  * Mapping things from DRM to DC and from DC to DRM is really confusing for
>    outside contributors like myself, so it wasn't even really clear to me if
>    there was another way to reconstruct the DRM context from the spots
> where
>    we call from DC up to DM (not a typo, see next point).
>  * These DC structs for MST are already layer mixing as far as I can tell,
>    just not in an immediately obvious way. While this struct itself is for DC,
>    there's multiple spots where we pass the DC payload structs down from
> DM to
>    DC, then pass them back up from DC to DM and have to figure out how to
>    reconstruct the DRM context that we actually need to use the MST helpers
>    from that. So, that kind of further complicates the confusion of where
>    layers should be separated.
>  * As far as I'm aware with C there shouldn't be any issue with adding a
>    pointer to a struct whose contents are undefined. IMHO, this is also
>    preferable to just using void* because then at least you get some hint as
>    to the actual type of the data and avoid the possibility of casting it to
>    the wrong type. So tl;dr, on any platform even outside of Linux with a
>    reasonably compliant compiler this should still build just fine. It'll even
>    give you the added bonus of warning people if they try to access the
>    contents of this member in DC on non-Linux platforms. If void* is preferred
>    though I'm fine with switching it to that.
> 
> --
> Cheers, Lyude Paul (she/her) Software Engineer at Red Hat

Hi Lyude,

Thanks for your time!
I was thinking that our DC just mainly takes care of HW related programming. 
Struct dc_dp_mst_stream_allocation is only used to construct a copy of the virtual 
channel payload ID and slots count of payload allocation table determined by
dm/drm. ID and slots are only things required for programming HW registers.
I think there shouldn't be any spots to try to construct the DRM context from
this dc struct since there is no such concept in HW level. Our HW should only 
take care of local DP link and it doesn't have overall topology info.

Thanks!

Regards,
Wayne