Re: Power saving using Display port HPD

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 10 Apr 2014, Daniel Vetter <daniel@ffwll.ch> wrote:
> 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@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@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.

Hmm, in my reply I was assuming this had happened. The dmesgs would
confirm.

Jani.


>
> 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
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-- 
Jani Nikula, Intel Open Source Technology Center