On Thu, 17 May 2018, John Sledge john_sledget@yahoo.com wrote:
I’ve been doing some PTN3460 programming under Linux using C/C++ and I have some questions regarding on setting the brightness level to my display device.
The display device with PTN3460 is connected in DP (display port) to my computer. Only needs a DisplayPort native AUX command to access DPCD address from PTN3460. I’m currently looking into the DRM (Direct Rendering Manager) a subsystem of the Linux kernel. It has a methods drm_dp_dpcd_readb, drm_dp_dpcd_read and drm_dp_dpcd_write.
Do you have any suggestions or advice how to use the kernel driver in DRM in regards to how to implement the method drm_dp_dpcd_readb for example? I couldn't not find any test tool examples that implement it. Biggest concern is I don't have sufficient knowledge where to start what to code using the DRM module.
Let me double check, you're talking about doing DPCD access from userspace? The *only* interface that can be recommended for that is the DRM DP AUX interface. If you have kernel config DRM_DP_AUX_CHARDEV=y, you'll get /dev/drm_dp_auxN node(s) that allows you to read and write arbitrary DPCD offsets. It's a chardev; you can use e.g. dd to debug read DPCD.
Of course, it would be better to have a generic backlight interface for DPCD based backlight in kernel. We have the basics for that for Intel GPU in i915/intel_dp_aux_backlight.c. Granted, it should be moved to common DRM code, but it also doesn't work for you if you have the chip connected to regular DP. It expects eDP, and somewhat spec compliant eDP DPCD backlight support.
HTH, Jani.