[Bug 96996] [BYT][VLV] No picture on DSI display after DRM loads on Teclast X80H tablet

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=96996

--- Comment #15 from Stephan Schorn <mail at schorn-stephan.de> ---
I had a look into the regdumps and compared the dmesgs again.
The GMBus frequencies are wrongly calculated on our devices.

There had already been problems with the GMBus on VLV which have been fixed
with this commit:

http://o.cs.uvic.ca:20810/perl/cid.pl?cid=24eb2d599b6a2bf7761c00e1959898d1d9240cb5

The regdump of the working 2.6.7 fastboot=1 regdump shows
GMBUS frequency binary encoding 
GMBUSFREQ (0x00180000:0x00006510): 0x0000014e
This is 334 in decimal

The regdump of the not working 4.9.0 shows
GMBUSFREQ (0x00180000:0x00006510): 0x0000010b
which is 267 in decimal.

The dmesgs show time out events of the GMBus. The wrong frequency could cause
them.


If you have a look into the commit or the regarding code, you'll see GMBus
frequency is calculated with the CDCLK which also showed strange values in the
dmesgs as I've already mentioned. By the way, I found this very helpful:
https://01.org/sites/default/files/documentation/intel_os_gfx_prm_vol10_-_display_0.pdf

working:
CZCLK_CDCLK_FREQ_RATIO (0x00180000:0x00006508): 0x000000bb
CDCLK Frequency encoding:                       1011
CDCLK Divide Ratio:                             6
CD Register Encoding (Decimal):                 11 
SKU200 VCO  800 (MHz):                          133
SKU266 VCO 1600 (MHz):                          267
SKU333 VCO 2000 (MHz):                          333
CD Qual Gen Ratio:                              11

not working
CZCLK_CDCLK_FREQ_RATIO (0x00180000:0x00006508): 0x000000eb
CDCLK Frequency encoding:                       1110
CDCLK Divide Ratio:                             7.5
CD Register Encoding (Decimal):                 14 
SKU200 VCO  800 (MHz):                          107
SKU266 VCO 1600 (MHz):                          213
SKU333 VCO 2000 (MHz):                          367
CD Qual Gen Ratio:                              14

In both cases, the CZCLK Frequency encoding looks like this:
CZCLK Frequency encoding:                       1011
CZCLK Divide Ratio:                             6
CZ Register Encoding (Decimal):                 11
SKU200 VCO  800 (MHt):                          133
SKU266 VCO 1600 (MHz):                          267
SKU333 VCO 2000 (MHz):                          333
CZ Qual Gen Ratio:                              11


So, in the working configuration, CZCLK and CDCLK are identical. In the not
working configuration, they aren't. GMBus frequency is calculated usind CDCLK.
Wrong GMBus frequency means time out means no screen control means no picture.

Still, I have no idea, WHY these values are wrong. The next step in the
investigation would be to find out why the CDCLK is set to a new value although
it shouldn't. At least I think it shouldn't...

If someone has an idea that would help or someone finds an error in my
conclusions, it would REALLY be appreciated if this someone told me!!!

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