Thank you Jani and Ville and others for your feedback on this approach. I think adding the current negotiated link rate , lane count and DSC status as part of a connector property is a great way to standardize this instead of every driver adding a debugfs for this same information.
Please find my comments for why OS would use this information as below: On Mon, Apr 13, 2026 at 6:30 AM Jani Nikula <[email protected]> wrote: > On Mon, 13 Apr 2026, Kory Maincent <[email protected]> wrote: > > On Fri, 10 Apr 2026 00:36:09 +0300 > > Dmitry Baryshkov <[email protected]> wrote: > > > >> On Thu, Apr 09, 2026 at 11:36:21PM +0300, Ville Syrjälä wrote: > >> > On Thu, Apr 09, 2026 at 07:08:16PM +0200, Kory Maincent wrote: > >> > > DisplayPort link training negotiates the physical-layer parameters > needed > >> > > for a reliable connection: lane count, link rate, voltage swing, > >> > > pre-emphasis, and optionally Display Stream Compression (DSC). > Currently, > >> > > each driver exposes this state in its own way, often through > >> > > driver-specific debugfs entries, with no standard interface for > userspace > >> > > diagnostic and monitoring tools. > >> > > > >> > > This series introduces a generic, DRM-managed framework for > exposing DP > >> > > link training state as standard connector properties, modeled after > the > >> > > existing HDMI helper drmm_connector_hdmi_init(). > >> > > > >> > > The new drmm_connector_dp_init() helper initializes a DP connector > and > >> > > registers the following connector properties to expose the > negotiated link > >> > > state to userspace: > >> > > > >> > > - num_lanes: negotiated lane count (1, 2 or 4) > >> > > - link_rate: negotiated link rate > >> > > - dsc_en: whether Display Stream Compression is active > >> > > - voltage_swingN: per-lane voltage swing level (lanes 0-3) > >> > > - pre_emphasisN: per-lane pre-emphasis level (lanes 0-3) > >> > > >> > I don't see why any real userspace would be interested in those (apart > >> > from maybe DSC). If this is just for diagnostics and whatnot then I > >> > think sysfs/debugfs could be a better fit. > >> > >> I'd agree here. Please consider implementing it as a debugfs interface, > >> possibly reusing the Intel's format. > > > > Sorry, I completely forgot to include a paragraph explaining the > rationale > > behind using DRM properties. > > > > This DisplayPort link information report was requested by OSes to allow > them to > > assess the capabilities of each DisplayPort connector on the system, and > to > > guide users from the most to least capable ones. It will also enable the > OS to > > warn the user when a cable is too long or experiencing noise (indicated > by high > > voltage swing and pre-emphasis levels). > > The selection of the number of lanes or link rate are at the discretion > of the driver, or link policy manager in DP spec terms. It does not > really convey the capabilities of the *connectors* but rather the > current *link*. Ditto for enabling DSC. > I agree that it would be up to each driver to have a policy on choosing link rate/lane count and when to enable DSC and then if it fails fallback to lower link rate/lane count as per the VESA spec. Currently this information is only available through a debugfs and not standard across the drivers. However adding it as part of the connector property would be useful for the userspace, such as DRM HWC to use this information along with the link status property to understand the reason for reduced resolutions/modes exposed to userspace. This can be used to provide better diagnostics and useful information up to the end user. DSC being enabled or not or what was the final negotiated link rate and lane count could be used to take smarter mode configuration handling choices in the usersapce For example for MST, if usersapce can know the negotiated link parameters usersapce can be smarter in choosing resolutions on each of the downstream sinks based on the total link bandwidth between source and the hub. At the end it just helps the OSs to provide better control to the user in certain scenarios. I agree that Voltage swing and pre emphasis values can be removed and we can focus on adding property for link rate, lane count and dsc info like enabled/disabled, compressed bpp etc. Regards Manasi > > I don't think the voltage swing and pre-emphasis are really diagnostic > measures either, but a response to measuring and adapting to the > link. And if the link training failed, the driver may have already > reduced the number of lanes and link rate to compensate. So you could > appear to have the perfect link only because it was so bad at high link > rate that it was reduced already. > > The policies may also vary from driver to driver, and possibly depending > on what makes sense for the hardware (e.g. power consumption with or > without DSC). > > I think "link information report ... requested by OSs" is vague, and I > don't think the concept has been completely thought through. I can't see > how you could present reliable and actionable information to the user > with what the patch at hand provides. Or how it could work in a generic > manner across drivers. > > Overall sounds like an XY problem [1]. We should focus on what you're > trying to achieve first, in userspace, and only then think about what > the appropriate kernel mechanism should be. > > I don't think this is it. > > > BR, > Jani. > > > [1] https://en.wikipedia.org/wiki/XY_problem > > > > > > Since this is information that OSes will consume on a regular basis, > exposing > > it directly as DRM properties seems the most appropriate approach. > > > > > > -- > Jani Nikula, Intel >
