[Intel-gfx] Power saving using Display port HPD

[Daniel Vetter]

On Thu, Apr 10, 2014 at 12:06:15PM +0530, Arun Chandran wrote:
> I have connected only a DP monitor on Bayley Bay platform and booting
> ubuntu 13.10 with the kernel
> http://cgit.freedesktop.org/drm-intel/log/?h=drm-intel-fixes.
> 
> I am experimenting with the HPD feature of display port with debug
> messages enabled. (drm.debug=0xe).
> 
> When I plug in/out DP cable I am able to see the below kernel messages
> and ubuntu desktop is displayed on the DP monitor
> 
> ##############################
> 
> 
> root at arun-valley-view:~# uname -a
> Linux arun-valley-view 3.14.0-00528-g67e9d82 #34 SMP PREEMPT Wed Apr 9
> 18:48:30 IST 2014 i686 i686 i686 GNU/Linux
> 
> root at arun-valley-view:~# dmesg -c
> 
> [ 1028.506188] [drm:intel_hpd_irq_handler] hotplug event received,
> stat 0x00100000
> [ 1028.506217] [drm:intel_hpd_irq_handler] Received HPD interrupt on
> PIN 5 - cnt: 0
> [ 1028.513858] [drm:i915_hotplug_work_func] running encoder hotplug functions
> [ 1028.513880] [drm:i915_hotplug_work_func] Connector eDP-1 (pin 5)
> received hotplug event.
> [ 1028.513896] [drm:i915_hotplug_work_func] Connector HDMI-A-2 (pin 5)
> received hotplug event.
> [ 1028.513913] [drm:_edp_panel_vdd_on] Turning eDP VDD on
> [ 1028.513931] [drm:_edp_panel_vdd_on] PP_STATUS: 0x80000008
> PP_CONTROL: 0xabcd000f
> [ 1028.515064] [drm:intel_dp_get_dpcd] DPCD: 11 0a 84 01 01 00 01 80
> 02 00 00 00 00 00 00
> [ 1028.516563] [drm:intel_dp_check_link_status] TMDS-22: channel EQ
> not ok, retraining
> [ 1028.530884] [drm:intel_dp_set_signal_levels] Using signal levels 00000000
> [ 1028.546113] [drm:intel_dp_set_signal_levels] Using signal levels 00000000
> [ 1028.547233] [drm:intel_dp_start_link_train] clock recovery OK
> [ 1028.548711] [drm:intel_dp_complete_link_train] Channel EQ done. DP
> Training successful
> [ 1028.549264] [drm:intel_dp_detect] [CONNECTOR:23:eDP-1]
> [ 1028.549761] [drm:intel_dp_probe_oui] Sink OUI: 00e04c
> [ 1028.550262] [drm:intel_dp_probe_oui] Branch OUI: 000000
> [ 1028.550343] [drm:intel_hdmi_detect] [CONNECTOR:21:HDMI-A-2]
> [ 1028.550595] [drm:gmbus_xfer] GMBUS [i915 gmbus dpc] NAK for addr: 0050 r(1)
> [ 1028.550611] [drm:drm_do_probe_ddc_edid] drm: skipping non-existent
> adapter i915 gmbus dpc
> 
> ##################################
> 
> After booting I am executing the below steps
> 
> 1) unplug DP cable.
> 
> 2) Mask the pin 18 (HPD) of DP cable using a paper strip then connect it
> back
>     I have checked the dmesg output and the there is no messages related to
> HPD. So kernel is
>     unaware that I have connected a DP monitor
> 
> But still I see ubuntu desktop coming on the DP monitor.  This means kernel
> does not stop pumping display data after step 1.
> 
> Is this the right behavior? . When kernel knows that the last existing
> display is removed (via the HPD feature) it can save power by stopping the
> data output to DP monitor (or shutting down the DP hardware related part.)
> 
> Does anybody knows how to use the HPD feature to save power? Or anybody has
> done this before?

Yes, this is expected behaviour. Userspace asked the kernel to shovel
pixels to the DP screen, the kernel obeyed. If you want to kill the pipe
when unplugged, you need to make that policy decision in userspace.

This is similar to how dpms off is driven by userspace and not the kernel.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch