x2APIC and a new device driver for Broadcom Fast Ethernet chips

Its been quite a while since I wrote something technical on my blog. I have been working on quite a few things off-late. Since my integration of x2APIC - a new Local APIC model which uses MSR (Model Specific Register) on future generation Intel Processors, I took a small challenge to work on Device Drivers and that too an Ethernet Controller. Having gained no knowledge about Networking and Device Driver in past years, I thought this is the time to jump-in. Better late than never you know. So this blog is really about two major things :-

x2APIC - A new Local APIC (Advance Programmable Interrupt Controller). It improves performance as the local APICs can write to registers parallely. With xAPIC (MMIO model), we use-to map local APIC registers in memory and hence any write to I/O space used to get serialize. x2APIC has some improvements in IPI (Inter-Processor Interrupt) too. It also extends support for Local APIC ID > 255 but I don't think any BIOS programs Local APIC ID > 255 as of now.

Broadcom Fast Ethernet (SUNWbfe) - This is a project which turned out to be a good experience. I had no prior knowledge of writing device drivers or Ethernet controllers. Initially, I was quite confused about the Ring-Architecture, Descriptors and Buffers. I was not able to fit everything in a big picture and convince myself that it works. I managed to learn about them after spending some two weeks looking for some documents on how TX/RX rings are organized. So the first thing was to document how a TX/RX ring is organized and it's well described here.

Solaris now have support for Broadcom 100-T-Base Fast Ethernet controller. It is a bit old Ethernet controller but a popular one. Moreover it makes lot of sense on Netbooks than laptops. This chip has only one TX and RX ring. The number of descriptors are programmable and it supports Multicast through CAM (Content Addressable Memory for 64 entries). It does not have support for Jumbo frame though and hence MTU is 1500. Having integrated bfe in Solaris Nevada the other day, my next target is to add support for Atheros/Attansic Ethernet controllers. They come in three flavors :-

- Atheros/Attansic L2 Fast Ethernet as device-id 0x2048

- Atheros/Attansic's AR8121/AR8113 PCI-E Ethernet Controller as device-id 0x1026

- Atheros/Attansic L1 Gigabit Ethernet 10/100/1000 Base as device-id 0x1048

The plan is to have support for all the three chips in atge (a new device driver or SUNWatge). I have started the work and I expect it to complete in two-three months timeframe.

/Saurabh

http://saurabhslr.blogspot.com