drm/panel: panel-simple power-off sequencing

[Jonas Mark (BT-FS/ENG1-GRB)]

Hi,

We have a parallel LCD panel which is driven by panel/panel-simple. The power-off sequence specified in the datasheet requires that the enable-gpio must be deasserted for a number of VSYNC cycles before shutting down all other control signals. See the diagram below:
                        __  __  __  __  __    
CLK, VSYNC, DE, HSYNC:  __><__><__><__><__\_____________________
                        ______
enable-gpio          :        \_________________________________

So far, in kernel 5.4 we relied on the unprepare delay time for making sure that the enable-gpio timing requirements are fulfilled. That is, the
panel_simple_unprepare() would:

1. Deassert the enable-gpio
2. Switch off the voltage regulator
3. Wait display_desc.delay.unprepare milliseconds

Afterwards the IPU was shutdown, and all the control signals stopped. 

But with the below commits:

 - 3235b0f20a0a4135e9053f1174d096eff166d0fb  
   "drm/panel: panel-simple: Use runtime pm to avoid excessive unprepare / prepare" 
 - e5e30dfcf3db1534019c40d94ed58fd2869f9359 
   "drm: panel: simple: Defer unprepare delay till next prepare to shorten it"

The enable-gpio is now deasserted in panel_simple_suspend(), which is called some time after the disablement of control signals are stopped:
                        __  __  __  __  __    
CLK, VSYNC, DE, HSYNC:  __><__><__><__><__\_____________________
                        __________________________
enable-gpio          :                            \_____________

With the latest panel-simple, is there a way which allows us to deassert enable-gpio before the control signals stop?

Mit freundlichen Grüßen / Best regards

Mark Jonas

Building Technologies, Panel Software Fire (BT-FIR/ENG1-Grb)
Bosch Sicherheitssysteme GmbH | Postfach 11 11 | 85626 Grasbrunn | GERMANY | www.boschsecurity.com
Tel. +49 89 6290-1233 | Telefax +49 89 6290-281233 | Mark.Jonas at de.bosch.com

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