[mythtv-users] Standard PCI VDPAU-capable Video Cards vs. PCIExpress

[Josh Mastronarde]

On Sun, Nov 28, 2010 at 7:26 AM, Brian Wood <beww at beww.org> wrote:
> I have several Socket 939 systems, which I would like to upgrade to be
> able to use VDPAU. They all have AGP and PCI slots, and no PCIExpress.
>
> I see two options:
>
> Replace the motherboards with ones that have a PCIExpress slot, and use
> PCIExpress VDPAU-capable video cards. I'd have to do that quickly, as I
> suspect Socket 939/PCIExpress mobos won't be available for too much
> longer.
>
> Use the current mobos, and standard PCI video cards that can do VDPAU.
>
> Obviously the second choice will save money, but I'm concerned about the
> limitations of using standard PCI video cards.
>
> The cost of the PCI and PCIExpress video cards is similar, so choice 2
> will only save the cost of the mobos, which is not that much. I really
> want to save the cost of replacing the CPUs and the RAM, which either
> option will do.
>
> So the real question is, what would I be giving up by using standard PCI
> video cards, is anyone using them with a Myth system, and what, if any,
> problems is it leading to? OSD problems? Speed problems when the CPUs
> are throttled?
>
> Any thoughts would be appreciated, as always.

Speaking as someone who worked on PCI and PCIExpress chipsets for a
major chip manufacturer (but who is prone to getting the calculations
wrong, so check them!):

Standard PCI runs at 33MHz x 32 bits (4 Bytes), for a max theoretical
bandwidth of 133MB/s -- but because the same channel is used for
commands and data in both directions, actual bandwidth will be much
lower.

When used to transfer compressed data to a VDPAU card, 133MB/s =
1Gbit/s, which should be more than enough for any content.  20Mb/s HD
content, for example, will only use ~2% of the max bandwidth, so there
shouldn't be any impact on other system uses (and in most current
systems, the expansion PCI slots are on a dedicated bus, so you're not
sharing that 133MB/s with as many of the system controllers as
originally).  CPU throttling shouldn't affect this, and I'd expect
(without doing any calculations) that there's enough bandwidth for OSD
drawing, etc.

However, when not using VDPAU and decoding in software to the frame
buffer, that same HD content takes 1920x1080x4 Bytes/pixel =
~8MB/frame, at 60fps = 486MB/, which is way beyond the PCI
capabilities.  So you're pretty much committed to VDPAU, whereas with
PCIExpress you'd be able to use software decode for any codecs that
the hardware can't handle.  I don't use VDPAU today, so don't know how
much of a risk this is.

I can't comment on Socket 939 itself, but as it's our competitor's
product I'm obligated to say that there are better solutions out there
:-)

Josh