• Lyude Paul's avatar
    drm/dp_mst: Use full_pbn instead of available_pbn for bandwidth checks · fcf46380
    Lyude Paul authored
    DisplayPort specifications are fun. For a while, it's been really
    unclear to us what available_pbn actually does. There's a somewhat vague
    explanation in the DisplayPort spec (starting from 1.2) that partially
    explains it:
    
      The minimum payload bandwidth number supported by the path. Each node
      updates this number with its available payload bandwidth number if its
      payload bandwidth number is less than that in the Message Transaction
      reply.
    
    So, it sounds like available_pbn represents the smallest link rate in
    use between the source and the branch device. Cool, so full_pbn is just
    the highest possible PBN that the branch device supports right?
    
    Well, we assumed that for quite a while until Sean Paul noticed that on
    some MST hubs, available_pbn will actually get set to 0 whenever there's
    any active payloads on the respective branch device. This caused quite a
    bit of confusion since clearing the payload ID table would end up fixing
    the available_pbn value.
    
    So, we just went with that until commit cd82d82c ("drm/dp_mst: Add
    branch bandwidth validation to MST atomic check") started breaking
    people's setups due to us getting erroneous available_pbn values. So, we
    did some more digging and got confused until we finally looked at the
    definition for full_pbn:
    
      The bandwidth of the link at the trained link rate and lane count
      between the DP Source device and the DP Sink device with no time slots
      allocated to VC Payloads, represented as a Payload Bandwidth Number. As
      with the Available_Payload_Bandwidth_Number, this number is determined
      by the link with the lowest lane count and link rate.
    
    That's what we get for not reading specs closely enough, hehe. So, since
    full_pbn is definitely what we want for doing bandwidth restriction
    checks - let's start using that instead and ignore available_pbn
    entirely.
    Signed-off-by: default avatarLyude Paul <lyude@redhat.com>
    Fixes: cd82d82c ("drm/dp_mst: Add branch bandwidth validation to MST atomic check")
    Cc: Mikita Lipski <mikita.lipski@amd.com>
    Cc: Hans de Goede <hdegoede@redhat.com>
    Cc: Sean Paul <sean@poorly.run>
    Reviewed-by: default avatarMikita Lipski <mikita.lipski@amd.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20200306234623.547525-3-lyude@redhat.comReviewed-by: default avatarAlex Deucher <alexander.deucher@amd.com>
    Tested-by: default avatarHans de Goede <hdegoede@redhat.com>
    fcf46380
drm_dp_mst_topology.c 149 KB