While the patch is entitled new driver for siliconmotion, it appears to be extending the existing xf86-video-siliconmotion DDX driver as opposed to writing a new driver from scratch. The e-mail itself is a bit of a mess, but there's a nearly 40,000 line patch that's attached.
The description on this work just comes down to "New driver works for SM712/722/502/750/718/750LE. This is the first submission." The patch itself can be seen in the mailing list archive from Silicon Motion's Aaron Chen.
The Silicon Motion SM712 is a PCI-based 2D graphics engine with 4MB of embedded video memory and a maximum output resolution of 1280x1024. This is rather poor by today's standards, but the PCI-based SM718 allows for 16MB of embedded memory and a 1920x1440 output. The SM750 SoC moves over to PCI Express while still carrying the 1920x1440 limitation and only 16MB of embedded memory. At least this is good enough for servers/embedded systems.
This new open-source driver support is nice if you're an existing Silicon Motion customer, but not too appealing otherwise. It would have been more interesting had the new open-source hardware enablement more timely after the product's debut and if this was a DRM/KMS-based driver rather than just the DDX driver.
From this past March, see Silicon Motion Has Open-Source Driver, But Fails.