fixing X for GeodeLX

[ link: fixing-x-for-geode-lx | tags: geode x linux x86emu ubuntu | updated: Wed, 30 Apr 2008 10:42:10 ]

Recently I have been doign a bit of contract work for Symbio Technologies. They have had me do various little projects part time. Most recently I got a chance to work on X.org video drivers for the Geode family.

Here is the progress...

  • starting X freezes the system

    With X-core release 1.3 (specifically Ubuntu/Gutsy, as that is what Symbio uses) there was a bug that caused a freeze of the system. This only occurred on some hardware, notably those systems with the GSW VGA BIOS.

    The first symptom that I could see was the freeze. I narrowed it down to X86EMU, an X subsystem that emulates the BIOS real-mode code in usermode to gain access to VGA BIOS features like monitor mode information (DDC). It turned out that the emulator was trying to write to an invalid I/O port, which disabled PIC interrupts.

    The first patch was triggered by a suggestion from Alan Cox that certain I/O ports are not to be touched from user space. These patches prevent X86EMU from accessing ports that Alan Cox said are not be accessed :)

    But why would the BIOS try to muck with the PIC when being asked for DDC info? Well it turned out that it wasn't. X86EMU choses not to allow drivers to access PCI directly, instead it goes through an emulation layer -- this is good -- but instead of emulating the access to arbitrary devices, it was always passing the requests to the VGA device that was in use by X -- this is bad. The BIOS was trying to read something from the PCI bridge, but X was returning values from the VGA device. The value was not validated, and caused PIC access.

    The second patch fixes the X86EMU PCI emulation bug. When the code emulated wants to talk to a certain device, instead of using the VGA device use what they asked for. The second patch here uses libpciaccess, while the first one does not.

    These patches were not yet picked up by upstream, but are in Ubuntu and possibly other distros.

    Ref: launchpad #140051 freedesktop #14332

  • switching back to virtual terminal doesn't work

    The problem here was in X86EMU again. The GSW VGA BIOS was issuing a CPUID instruction to perform some operation required for a VT-switch. The patch implements a CPUID instruction in X86EMU by returning raw values from the CPU, or the 486dx4 values on non-x86 processors.

    This patch was universally picked up by pretty much everyone because it's trivial.

    Ref: launchpad #180742

  • wide mode not working

    Next problem was a long time wish of the Geode users; to have wide monitors supported in native resolutions. The problem here was with the way that the resolutions are filtered by LXValidMode() against a table of valid modes. Something that is not required if the modes come from DDC and/or we are not using a PANEL (ie using a monitor connected by VGA cable).

    Jordan and I colaborated on this. In the end he produced a patch that I fixed a minor type-o bug in and tested...

    Ref: launchpad #197069

    Git: 1e763626a

    Packages in my Ubuntu PPA

If you have any questions about the above, give me a shout.

Bart's Blog

/fixing-x-for-geode-lx

About

I am an embedded Linux software developer and consultant operating under Jukie Networks Inc in Ottawa, Canada.

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first post!




Bart Trojanowski
http://www.jukie.net/~bart
bart@jukie.net