[Bug 26614] New: Radeon driver clears TMDS register

[bugzilla-daemon at freedesktop.org]

http://bugs.freedesktop.org/show_bug.cgi?id=26614

           Summary: Radeon driver clears TMDS register
           Product: xorg
           Version: 7.1
          Platform: x86 (IA32)
        OS/Version: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/Radeon
        AssignedTo: xorg-driver-ati at lists.x.org
        ReportedBy: rmacdona at ati.com
         QAContact: xorg-team at lists.x.org
                CC: cooper.yuan at amd.com


The ATI video chip RN50 (aka ES1000) provides digital video via external TMDS
as well as standard VGA output.  The digital video is a 24 bit parallel video
output plus 4 bits for control signals.  This is occasionally termed DVO - for
Digital Video Out.

We have found that the Radeon driver provided with SLES 10 and RHEL 5.2
inappropriately clears the disp_hw_debug register which is used to set the
output impedance of the buffers for the DVO signal.  This register is at offset
0xD14 on the RN50. By setting this register to zero the Radeon driver causes
these signals to be too weak for some DVO receivers.  

The disp_hw_debug register is set by the RN50 video BIOS and should not be
modified by the Radeon driver.

To verify this:  

1.  Boot to Runlevel 3.  Read the register.  It has the non-zero BIOS settings.
2.  startx, read the register again.  It is now 0.

The problem is seen on some KVM systems that use the RN50 digital output.  Due
to the nature of the signals this is only a problem for resolutions above
1280x1024.


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