Re: atombios_crtc.c - make error messages distinguishable "unable to allocate a PPLL\n"

["Christian König" <christian.koenig@amd.com>]

On 11.06.2015 00:12, James Feeney wrote:
>> Yes, DP needs its own clock source.  However, you only need one clock source
>> for all DP ports so you can drive multiple DP displays from a single clock
>> source.  The clock source used for DP cannot be shared with non-DP displays.
>> ...
>> There are additional caveats however.  For example, you can't
>> share a clock source between two displays with different hdmi audio settings
>> even if the mode clocks are the same
> Again, thanks for that.  That's quite helpful.  In extensive reading, trying to
> understand what would be required to run three displays, I saw no mention of
> those two points.
>
> If the radeon driver could report a) what architecture it sees/code path it is
> following, b) the total number of clock sources found, and c) which clock
> sources are assigned to which outputs, then it would be much easier to discover
> the current display configuration and to anticipate what display configurations
> would be "workable".

Yeah, that's indeed true. We already dump the connector configuration on 
bootup, but not the config of the PLLs driving them.

Feel free to submit a patch for this, it clearly sounds like a good idea.

>
> The radeon driver can be quite "obstinate" when trying to set a specific
> configuration for three displays, where the result will depend upon the
> configuration history, or perhaps is strongly dependent upon the relative
> position of the displays.
>
> For instance, with my HD 5570, and supposing that the external clock is not
> installed, and that there are still two independent internal clock sources
> available, it should be possible to assign one clock source to the DisplayPort
> output, and one clock source shared to the DVI and HDMI outputs.  If I change
> the output modes from a working configuration to, say, a test configuration, and
> then try to change back again, some repeated "trial and error" attempts with
> xrandr are required, before the original configuration will "take".  Often, the
> first reconfiguration or two results in some black screens, and then "something"
> in some subsequent xrandr reconfiguration will trigger a screen update where the
> displays are first "scrambled" and then are redrawn correctly.
>
> This suggests to me some kind of "hidden state" in the radeon driver, which is
> being updated by some indirect mechanism when I change output modes.  Sorry, I'm
> not being precise.  Does anyone have any thoughts about how to better understand
> this reconfiguration sequence, and how to make the process more "robust" and
> predictable?

Well, the basic problem is the hardware. And the "hidden state" you are 
talking about is just the hardware state.

Especially on older generations you have all kind of funny dependencies 
in the PLLs that you need to take care of in the software.

Some have special routing requirements, e.g. can drive only certain 
outputs, some can only generate a certain range of frequencies 
(resolutions), some have undocumented power management dependencies 
(e.g. you turn PLL1 off and PLL2 suddenly becomes unstable).

The last time I tried to fix some corner cases I've broken a lot of 
other users with my patches. So a very typical scripting comment applies 
here as well: "It works don't touch it"! We could try patches, but the 
chance of fixing it for one person and breaking it for somebody else is 
just very high.

The best advice I can give you if you want to use more than two displays 
at the same time make sure to have DP displays, and that's a good idea 
no matter what hardware you have.

Regards,
Christian.

>
> James

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