[Bug 91861] New: [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[Bug 91861] New: [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

            Bug ID: 91861
           Summary: [Radeon RS780] Blank screen (no signal) on HDMI after
                    boot in 3.15 & later
           Product: Drivers
           Version: 2.5
    Kernel Version: 3.15 + later
          Hardware: Intel
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: michael.selig@fs.com.au
        Regression: No

Created attachment 164451
  --> https://bugzilla.kernel.org/attachment.cgi?id=164451&action=edit
dmesg (kernel 3.18.3) - RS780 blank screen

I use this machine as a HTPC connected to our TV via HDMI. It has been running
Ubuntu of various versions using the RADEON driver with no problems until
Ubuntu 4.10 (kernel 3.16), when this problem first showed itself. I can
successfully boot 3.13 (from Ubuntu 4.04), but when it was announced that VDPAU
was supported on this old Radeon hardware from 3.18, I was hoping that by
upgrading to 3.18, it would not only solve the "blank screen" problem, but also
give me better video performance on XBMC/KODI. Unfortunately 3.18 exhibits the
same "blank screen" problem. I then tried installing other kernel versions
(including 3.19rc5), but nothing from 3.15 onwards works.

The RS780 hardware is integrated on the motherboard, and is fast enough for
videos & TV etc, so I don't really want to change it for a noisy and/or heat
generating video card.

My symptoms are that, after seeing the Grub screen, I see a kernel message or 2
(nothing important) flashing up for a fraction of a second, but then it goes
blank, saying "no signal". I do not see the normal purple Ubuntu splash screen,
which appears before Xorg starts. Interestingly, on occasions it will boot 3.18
successfully, but that is usually after a reboot from 3.13 or 3.14. Once it
boots successfully it seems fairly stable, and I have managed to watch videos
and TV on 3.18 a couple of times though I haven't tested it for long. However
if it is booted again, chances are that it will fail. When the screen is blank,
the system is still up - I can SSH to the box fine. And it seems that Xorg is
up and running fine - Xorg.0.log looks fine - it's just that I can't see
anything!

I have tried radeon.runpm=0 radeon.dpm=0, but they make no difference.

I think this problem may be related to
https://bugzilla.kernel.org/show_bug.cgi?id=83461.

Any ideas?

Thanks,
Mike

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #1 from Mike S. <michael.selig@fs.com.au> ---
Sorry, the Ubuntu versions should be 14.04 & 14.10 of course.

Mike

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

Alex Deucher <alexdeucher@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |alexdeucher@gmail.com

--- Comment #2 from Alex Deucher <alexdeucher@gmail.com> ---
(In reply to Mike S. from comment #0)
> I think this problem may be related to
> https://bugzilla.kernel.org/show_bug.cgi?id=83461.
>

Probably the same issue.  Do any of the patches on that bug help?

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #3 from Mike S. <michael.selig@fs.com.au> ---
I have never built Linux before, but I'll  give it a go. It might take me
several days.

If I might make a suggestion, it would be nice you could set those divider
values as boot params.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #4 from Mike S. <michael.selig@fs.com.au> ---
I haven't yet tested the patch to cap ref_div to 7, but here are the values
reported with drm.debug=0xE, in case they are useful:

1. Kernel 3.14 (working):
14851, fb: 145.2 ref: 2, post: 7

2. Kernel 3.18 (not working):
148500 - 148500, fb: 1016.3 ref: 14, post: 7

I am no expert, but looking at the code, a ref_div value of 14 seems wrong. In
r600d.h the REF_DIV_MASK is only 3 bits wide, so the max value it should ever
be is 7, right?

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

Christian König <deathsimple@vodafone.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |deathsimple@vodafone.de

--- Comment #5 from Christian König <deathsimple@vodafone.de> ---
(In reply to Mike S. from comment #4)
> I am no expert, but looking at the code, a ref_div value of 14 seems wrong.
> In r600d.h the REF_DIV_MASK is only 3 bits wide, so the max value it should
> ever be is 7, right?

The REF_DIV_MASK in r600d.h is for the system clock setup, not for the display
clock.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #6 from Mike S. <michael.selig@fs.com.au> ---
I can confirm that the patch
https://bugzilla.kernel.org/attachment.cgi?id=163891 to cap ref_div to 7 fixes
the problem on my RS780. I did the test on kernel 3.18.3. Note that I also cast
the number 7 in the min() to unsigned to avoid a compiler warning.

I'd like to run it for a few days to ensure that it is stable.

I'd also like to ask a question, please. With this patch, the GPU divider
values are now running at:

148500 - 148490, pll dividers - fb: 580.7 ref: 7, post 8

while on 3.14 they were:

14851, pll dividers - fb: 145.2 ref: 2, post: 7

These new values produce a marginally different dot clock rate (the old debug
msg displayed the value 10x less than the new one), but the difference is tiny,
and probably doesn't make any practical difference.

Other than the dot clock, do the values of the 3 dividers affect anything else
in the GPU? Are there any advantages or disadvantages running the FB and REF
substantially higher (as they are now)?

Thank you!

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #7 from Christian König <deathsimple@vodafone.de> ---
(In reply to Mike S. from comment #6)
> I can confirm that the patch
> https://bugzilla.kernel.org/attachment.cgi?id=163891 to cap ref_div to 7
> fixes the problem on my RS780. I did the test on kernel 3.18.3. Note that I
> also cast the number 7 in the min() to unsigned to avoid a compiler warning.

Thanks for testing.

> I'd also like to ask a question, please. With this patch, the GPU divider
> values are now running at:
> 
> 148500 - 148490, pll dividers - fb: 580.7 ref: 7, post 8
> 
> while on 3.14 they were:
> 
> 14851, pll dividers - fb: 145.2 ref: 2, post: 7
> 
> These new values produce a marginally different dot clock rate (the old
> debug msg displayed the value 10x less than the new one), but the difference
> is tiny, and probably doesn't make any practical difference.

It does make quite a difference, that's why I've added the new code in the
first place. Some people had problems with the old one resulting in unstable
signals and audio/video desync after a while.

> Other than the dot clock, do the values of the 3 dividers affect anything
> else in the GPU? Are there any advantages or disadvantages running the FB
> and REF substantially higher (as they are now)?
When you use higher dividers in a PLL you can get closer to the desired
frequency, but at the cost of higher jitter. The trick is to stay within the
limits of the PLL. For example the VCO shouldn't get to fast otherwise the
input voltage gets to high, the feedback divider shouldn't get to high cause
otherwise the jitter is to high for the PLL to follow etc...

We most likely have an undocumented limit on the input frequency here,
otherwise limiting the reference divider won't have such an effect.

Christian.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #8 from Mike S. <michael.selig@fs.com.au> ---
Thanks for the explanation.

Can I suggest that you delay committing this patch for a week or so - I want to
check that it is really stable. Yesterday I had one incident where the screen
froze with diagonal coloured stripes for a couple of seconds, which I have
never seen before. It recovered itself OK without having to re-boot. Could this
have been caused by the higher divider values?

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #9 from Alex Deucher <alexdeucher@gmail.com> ---
(In reply to Mike S. from comment #8)
> Thanks for the explanation.
> 
> Can I suggest that you delay committing this patch for a week or so - I want
> to check that it is really stable. Yesterday I had one incident where the
> screen froze with diagonal coloured stripes for a couple of seconds, which I
> have never seen before. It recovered itself OK without having to re-boot.
> Could this have been caused by the higher divider values?

Probably not.  Usually PLL problems manifest themselves as no signal or
periodic signal drop outs.  Keep in mind that prior to kernel 3.15, the driver
used a different algorithm that had its own limitations.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #10 from Christian König <deathsimple@vodafone.de> ---
(In reply to Alex Deucher from comment #9)
> (In reply to Mike S. from comment #8)
> > Thanks for the explanation.
> > 
> > Can I suggest that you delay committing this patch for a week or so - I want
> > to check that it is really stable. Yesterday I had one incident where the
> > screen froze with diagonal coloured stripes for a couple of seconds, which I
> > have never seen before. It recovered itself OK without having to re-boot.
> > Could this have been caused by the higher divider values?
> 
> Probably not.  Usually PLL problems manifest themselves as no signal or
> periodic signal drop outs.  Keep in mind that prior to kernel 3.15, the
> driver used a different algorithm that had its own limitations.

Yeah, agree. For PLLs you usually know in a few minutes if your setup works or
not.

The only thing that could affect it a bit is the temperature, but then you are
driving the PLL so close to the edge that you will notice problems relatively
soon as well.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #11 from Mike S. <michael.selig@fs.com.au> ---
Thanks for the info. It hasn't happened again so far.

> ... you are driving the PLL so close to the edge ...
Can you please explain what you mean? Are you saying that the higher fb_div &
ref_div are pushing the PLL harder or faster?

Please bear in mind that I am a programmer, not an electrical engineer, and
though I do understand the basic principles of video displays (dot clocks,
refresh rates, blanking etc), to me these dividers are just numbers used to set
the dot clock.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #12 from Christian König <deathsimple@vodafone.de> ---
(In reply to Mike S. from comment #11)
> Thanks for the info. It hasn't happened again so far.
> 
> > ... you are driving the PLL so close to the edge ...
> Can you please explain what you mean? Are you saying that the higher fb_div
> & ref_div are pushing the PLL harder or faster?

If you want details you should probably read them up on Wikipedia, but I will
try to explain it in a few sentences.

The basic problem is how to generate a stable but still programmable frequency
in electronics.

On the one hand you have crystals which are usually very stable over a long
period of time, but to change the frequency of a crystal you would need to
change it's size and/or the material it is made of. Clearly not something you
can do with software.

On the other hand you have voltage controllable oscillators (VCO), which have
the nice feature that you can push a certain voltage into them and get a
certain frequency in return. Problem with those is that they are not
temperature stable, e.g. you set up a certain voltage to get 100Mhz and after a
while the electronics has warmed up and you suddenly get 101Mhz or 99Mhz or
something like this.

The solution is a PLL, it compares a very stable input frequency to a frequency
generated with a VCO and based on the difference adjusts the input voltage of
the VCO. This way the VCO frequency is slowly adjusted to the stable input
frequency and also stable over time when the electronics warms up.

Imagine that you now put a counter between the VCO output and the comparator
input that counts to 3 before forwarding the VCO frequency to the comparator.
This way when you input a frequency of 100Mhz you get 300Mhz as output
frequency. This counter is easily adjustable with software and called the
feedback divider.

So in the end you've got 300Mhz. But what do we do when for example we want
150Mhz? For this the electronics got a so called post divider it's also
software configurable and when for example when you have 300Mhz VCO frequency
and set the divider to 2 you get 150Mhz at the PLL output.

The voltage given as input to the VCO needs to be in certain limits. The upper
limit is easily understandable imagine that you push 20V into a 5V circuit all
you usually get is blue smoke and useless electronics. The lower limit usually
isn't so easily explainable, just keep in mind that the electronics need a
certain voltage to start up. More rarely you also got voltage gaps where the
VCO won't produce a stable output etc...

So to stay within those limits you add a third divider which is put between the
input frequency and the comparator. This divider is called the reference
divider.

Driving the PLL at the edge now means that at some point the VCO is to close to
one of it's limits. E.g. when it startup the VCO usually doesn't match the
input frequency at all and because of this the voltage input jumps around quite
a bit. So it sometimes works and sometimes doesn't. Or it initially works but
after the electronics has warmed up the VCO is suddenly out of range etc
etc....

Maybe that were a few more sentences than I expected, hopefully it was still
understandable.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #13 from Mike S. <michael.selig@fs.com.au> ---
Wow, thank you very much for the detailed explanation :-)

Though all I was really expecting was a "yes/no or not sure" answer to  whether
the new higher values of fb_dev & ref_div on my RS780 could be responsible for
pushing the PLL "closer to the edge" than with the driver on kernel 3.14, and
therefore maybe less stable and/or produce more heat.

So, from your explanation, using a ref_div of 7 compared to 2, means that the
VCO is running at a higher frequency, and hence, higher voltage. So I assume
that would mean the GPU would be drawing more power & therefore run hotter,
though I am not sure how significant this is to its overall power use.

Is that right?

The reason I would like to know this is to determine whether the new divider
numbers would cause the GPU to be less stable & hotter, which is fairly
important in a HTPC where I am trying to keep fan noise to a minimum. If so, I
might then try to force the ref_div below 7 in the driver (as I know it has
been stable for several years at 2).

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #14 from Christian König <deathsimple@vodafone.de> ---
(In reply to Mike S. from comment #13)
> So, from your explanation, using a ref_div of 7 compared to 2, means that
> the VCO is running at a higher frequency, and hence, higher voltage. So I
> assume that would mean the GPU would be drawing more power & therefore run
> hotter, though I am not sure how significant this is to its overall power
> use.
> 
> Is that right?

No, just the other way around. A higher reference divider results in a lower
post divider and a lower feedback divider and that in turn results in a lower
voltage and lower power consumption.

But the over all power consumption of a PLL compared to the whole system or
even the GFX block is completely neglitable. I would guess something between a
1/1000000 and 1/100000 of a Watt compared to a couple of Watt for the whole
system.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #15 from Mike S. <michael.selig@fs.com.au> ---
Thanks again for the info.

I have been running the Radeon driver with the patch to limit the reference
divider to 7 for a week now, and I can confirm that apart from that original
incident, it has been very stable.

So thank you for the patch, and I hope it can get into 3.19.

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

--- Comment #16 from Christian König <deathsimple@vodafone.de> ---
(In reply to Mike S. from comment #15)
> So thank you for the patch, and I hope it can get into 3.19.

Alex has added it to his -fixes branch, so it either ends up in 3.19 or in
3.20.

Can you close the bug report then?

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[Bug 91861] [Radeon RS780] Blank screen (no signal) on HDMI after boot in 3.15 & later

[bugzilla-daemon]

https://bugzilla.kernel.org/show_bug.cgi?id=91861

Mike S. <michael.selig@fs.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |CODE_FIX

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