[Bug 172421] New: radeon: allow to set the TMDS frequency by a special kernel parameter

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=172421

            Bug ID: 172421
           Summary: radeon: allow to set the TMDS frequency by a special
                    kernel parameter
           Product: Drivers
           Version: 2.5
    Kernel Version: 4.8.0-rc7+
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri at kernel-bugs.osdl.org
          Reporter: estellnb at elstel.org
        Regression: No

Despite different claims by ATI in 2016 Radeon R5 230 graphics cards featuring
HDMI, DVI and VGA had originally been sold as 4K-ready up to the year 2015. The
only prove I have for this is a comercial invoice from Nexus Mobile and a
packaging card. As far as I know Radeon R5 230 cards currently sold can still
be run stable and reliable under UltraHD provided that you apply the right
kernel patch (see for the attachement). Unfortunately neither my former
merchant nor ATI have responded about my questions concerning this change in
trade and marketing policy.

  My request would now be to integrate the provided patch into the mainline
kernel. It does not change the behaviour of the radeon driver unless you
specify a nonzero value for the radeon.hdmimhz parameter. If you do I have
tested the R5 230 cards I have to run stable and reliably for days. At least
the 2GB variant of this card has largely sufficient resources for proficient
desktop computing under UltraHD including image manipulation in 3840x2160.

  While never officially discussed for the radeon driver nouveau is already
implementing a similar parameter called nouveau.hdmimhz since kernel 4.5.x.
Though it thereby becomes possible to specify a hdmimhz that is far above the
cards technical possibilties the nouveau developers I have talked with say that
it would rarely be possible to damage any card by overcloking the TMDS. In deed
I have successfully been overclocking my GeForce 9600M GT to feature 4K/2160p. 

  Even specifying values considerable higher than 225MHz did not damage my
GeForce 9600M GT though the screen stayed black upon the nouveau driver
initialization. While the Radeon R5 230 works well at 297MHz (as long as you
specify that via radeon.hdmimhz) I have similarly to the GeForce 9600M GT tried
to overclock a Radeon R7 240. It did produce stable images at a higher hdmimhz
like 330 though the HDMI input of my monitor features no more than 30Hz at
3840x2160 (tested with or without a DP-adapter).

  While it remains questionable if the provided patch can improve things for
newer Radeon cards I would believe it to be beneficial for some elder cards. At
least it is known to be beneficial for the R5 230 initially marketed as
4K-ready. The according radeon patch provided with this report has so far
already been accepted by the Mageia 6 distribution. Though the attached patch
is for application at the current 4.8.0-rcX+ kernels most of my machines that
rely on it still run with 4.6.0.

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