SMBus on PCI-Express

Comments and questions to the author of the "Down to the TLP" posts (Eli Billauer)

SMBus on PCI-Express

Postby Guest » Fri Mar 15, 2013 4:22 pm

Hello,

On what kind of real existing PCI(e) add-in cards are SMBus-Devices implemented and with what kind of functionality is the SMBus used on this PCI(e) add-in cards?
The latest Specification with text about the SMBus is for the conventional PCI (many years ago), in all further specifications is no text about the SMBus but the CEM-Spec for PCI-Express defines the 2 pins for the SMBus.
Is the SMBus on PCI(e) add-in cards obsolete today?

Regards,
Erik
Guest
 

Re: SMBus on PCI-Express

Postby support » Fri Mar 15, 2013 9:02 pm

Indeed, I've never seen them getting any attention in the papers I've read on PCIe, and the random FPGA development boards I just checked up weren't decisive about whether to connect them to the FPGA or not: Xilinx' SP605 and ML605 don't bother to connect these lines to anything. Altera's official boards for Stratix IV and V do connect them directly to the FPGA.

If and which motherboards connect this bus to some controller is also a question.

In short, I wouldn't be happy to rely on this bus in a real-life project. Maybe others have more experience with this.
support
 
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Re: SMBus on PCI-Express

Postby Guest » Sat Mar 16, 2013 7:33 pm

Hello eli,

thank you for your fast response, and sorry for my delay. ;)

If and which motherboards connect this bus to some controller is also a question.
All modern chipsets include one or more SMBus-Host-Controllers so it is very likely that the most motherboards has this 2 lines on their PCB. The Linux-Kernel does support all of the currently used SMBus-Host-Controllers, the needed infrastructure is available but the documentation say it is not frequently used and so not well tested. (it seems the SMBus-Infrastructure of Linux is mostly used for plain I²C and some private use cases)

My question comes from an other direction, i must develop a kind of mainboard with 2 or 3 PCI-Express-Slots and now is my question: "how likely is the SMBus used with add-in cards and for what functionality?" that means "what is the potential benefit of supporting the SMBus?". The Slots should be DC-isolated from the mainboard and this makes the implementation of SMBus very complicated.
The answer that i will prefer is "SMBus is obsolete for today PCI(e) add-in cards, the 2 pins can be unconnected without the risk of loosing any relevant/pertinent feature." but if exist a real benefit (with commonly used add-in cards) by supporting the SMBus then it can be a real reason for paying the additional costs for this feature.

Maybe others have more experience with this.
I hope that, too. Additional answers are very welcome.
The decision must not be taken in the next few days.

Regards,
Erik
Guest
 


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