Friday, February 09, 2007

The Retelling of a Myth: The OS

Now that I have built my Mini-ITX box, it was time to install the OS.

The platform I had chosen would be a bit more difficult than installing on a vanilla x86 machine. Since I was using a VIA EPIA board, this was choc-full of coprocessors, crazy chipsets and the like. The C7 1GHz processor would be ample to do most things, but I needed to ensure that everything ran comfortably in 512MB and didn't otherwise sap the juice from my CPU.

The biggest issue would be the S3 Unichrome graphics chipset. It absolutely had to work, since I needed MPEG2 acceleration and XvMC, not to mention TV-out. VIA has recently decided to open up the source to its Linux drivers, and since then the OpenChrome project has released the open-source versions that most distributions use. It appeared several had success with previous iterations, so I felt confident the C7 could be done as well.

Ubuntu is an extremely nice and light-weight distro. Since it can run on a minimum of packages, I decided to try it out first. MythTV packages abounded, and I was able to install everything quite easily. Ubuntu had decent support for Unichrome, and evidently installing the X11 drivers was just an apt-get away. However, actually building the X11 xorg.conf configuration file proved to be way too much of a headache. I love Ubuntu, don't get me wrong. But they need some decent administrative tools. I can hack an xorg.conf if I need to, but dammit I just don't have the time anymore. I was still able to glean some good information from those who did.

I eventually settled with OpenSUSE 10.2, which also has ready-made repositories for MythTV and OpenChrome (including a new repository by openSUSE itself). Of course at this point I have no DVD or CD to boot off of - I didn't purchase a drive to go with the box. Luckily with SuSE you can easily install without a CD... instead I took my bargain-bin 32M USB stick and built a USB boot disk. I was able to connect to a repository over the Internet, perform an install (albeit with GRUB errors) and come back the next morning with SuSE nearly ready to go.

One problem during USB installation is that the installer believes your USB stick is installable media - so GRUB becomes confused and assumes your hard drive is a secondary device. It's not of course... so when the SuSE installer attempts to install GRUB to your system several errors come spewing back to you. At this point you have no choice but to ignore them and press on. After the installer boots your system, you need to boot off your USB drive once again then, via the boot disk's installer menu, tell the disk to "Boot [the] Installed System." Afterwards your previous installer can resume & complete, at which point you can manually fix GRUB. For me, this meant opening up /boot/grub/device.map and removing /dev/sda as a mounted device. My primary IDE HD - /dev/hda - became the primary device for GRUB. After that was tweaked, I jumped into /boot/grub/menu.lst and made the appropriate changes whenever I saw hd0 or hd1. I re-ran grub-install and things booted swimmingly afterwards.

I followed the advice of ExtremeTech and created two partitions: one smaller 8 GB partition formatted with ext3, and a second XFS partition that occupied the remainder of the drive. The ext3 partition would be used for the OS and all system files, while the XFS /video partition would be used to store the large MPEG2 files that Myth would be generating. XFS works best with large files, and reportedly has faster write speeds than JFS. While it may not be as failsafe as ext3's journaling capabilites, I wasn't as worried about losing a recorded CATV stream as much as I worried about read/write performance.

There were a few minor hacks that appeared to be needed with earlier versions of openSUSE, but I'm not sure if they're needed anymore. I applied 'em anyway.

We move on to the Unichrome drivers next. Installing the OpenChrome drivers will do just fine... as long as you're using DVI or VGA outputs on the board. TV-out is a different animal altogether. The EPIA CN10000EG uses a VT1625 TV-out chipset, which currently isn't supported by the OpenChrome drivers. I spent nearly 20 hours trying an unbounded number of modelines and configuration tweaks in my xorg.conf, all to no avail. I ended up downloading and attempting to install VIA's own Linux drivers, but their installer never installed things correctly. It appeared to be looking for old XFree86 directories and files... ones that had since gone the way of the XOrg. I eventually had to tell VIA's installer to extract the files, build what it needed but then not to clean up its temporary installation directory with --keep. Once I had all the files to peruse, I was able to take XServer/via_drv.so from the installation script and manually copy it over to /usr/lib/xorg/modules/drivers/. Once I moved VIA's own drivers over and followed the expert advice of those before me, TV-out finally worked. I used the modeline's that VIA's installer automagically inserted in my xorg.conf file - the standard list of built-in modelines didn't seem to fill the entire screen. Finally, three days of tearing my hair out came to an end.

Unfortunately, I was never able to get XvMC successfully installed. I specified the driver library (in this case libviaXvMCPro.so in the config file /usr/etc/X11/XvMCConfig. For the life of me I don't know why Myth looks in the /usr/etc directory instead of /etc - so I just made a symbolic link between the two. Nevertheless, the VIA drivers didn't seem to provide the necessary XvMC libraries... even though the OpenChrome packages did. Maybe once OpenChrome catches up I'll be ablet o use their X11 driver and enable XvMC support for better MPEG2 acceleration and motion compensation... until then my workaround is disabling glx in xorg.conf (so that I didn't get lags from weird cache timeouts) and enabling kernel deinterlacing in MythTV (more on that later). Even without XvMC, however, I'm only using 40-45% of the CPU decoding MPEG2 files. No complaints there.

The VIA installer script also had extracted libddmpeg.so along with the X11 drivers. Not sure if it helped... but I replaced the existing /usr/X11R6/lib/libddmpeg.so with this version. We'll see if it breaks anything.

Next I needed to improve hard drive performance. UltraDMA/100 was enabled for my old WD 40GB drive, but performance from hdparm -Tt was still slow. The ATA drivers that shipped with SuSE's default kernel worked passably, but they weren't fast enough to read/write large MPEG2 streams. It turns out that there is a known issue with openSUSE 10.2 and VIA's VT8237 southbridge doesn't perform appropriately with it. Again I had to return to VIA's driver site and download a kernel patch to fix SuSE 10.2's ATA support. Once I installed the kernel sources, patched the appropriate files, rebuilt the kernel modules and reinstalled kernel/drivers/ata/libata.ko and kernel/drivers/ata/sata_via.ko. Once the modules were reloaded my IDE speed problems cleared up.

Sound would sometimes work, sometimes not. Occasionally sound would skip, crackle and pop with horrible results due to interrupt conflicts. I modified my GRUB boot parameters to include pci=noapic so that ACPI wouldn't allocate the IRQ's for the sound card. After that, sound worked consistently fine.

Now to give Myth a little extra juice, let's tweak the Myth frontend to run with higher priority. Just modify /etc/security/limits.conf and add the entries
mythtv     -      rtprio       50
and
mythtv     -      nice         0
, if mythtv is the user logging in and launching the front-end. This should give Myth higher priority over other system processes, just in case resource contention should become a problem.

Aight... VIA's chipsets were now covered... graphics, TV-out, MPEG2 and ATA works. Now time to turn our attention to the PVR-150.

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