RE: How to determine the speed of the PCI bus attached?

[lecos@samsung.co.kr]

> On Thu, 2 Nov 2000, Ingraham, Andrew wrote:
> 
> > > Is there any bits in the configuration space that we
> > > can read to determine the speed of the PCI bus that a
> > > device is attached to? The 66MHz Capable Bit (Bit 5 in
> > > the Status register)from what I understand only
> > > showing the capability of the device as a 66MHz device
> > > but the PCI bus it attached not necessary to be
> > > running at 66MHz. So is there any way that we can
> > > determine the speed of the PCI bus that a PCI device
> > > is attached to?
> >  
> > Use the M66EN signal, pin 49B on the PCI connector.
> > 
> > See section 7.5.1 in the PCI specs, revision 2.2.
> > 
> > The M66EN pin tells "whether the bus segment is operating at 66 or 33 MHz"
> > (which really means 33-66 MHz mode or 0-33 MHz mode).  If any device on the
> > bus, including the host, is not 66MHz-capable, M66EN is grounded, and the
> > bus will be in 0-33 MHz mode.
> > 
> > "If a 66 MHz PCI agent requires clock speed information ... it is permitted
> > to use M66EN as an input."
> 
> Ok for the explanation about how PCI controllers must behave regarding the
> M66EN pin or use it from hardware, but M66EN is not readable as a bit from
> the configuration space of PCI devices. Instead it seems that there is no
> generic way to know from software if the PCI BUS is operating between 0-33
> MHz or beyond 33 Mhz (i.e. with 66MHz timing requirements instead).
> 
> As a result, the only way for the software to get such information could
> be to have some device on the BUS that tells you about that through some
> IO register, or that implements some timer based on PCI clocks so that you
> can implement some algorithm to estimate the PCI clock frequency.
> 
> Gerard.

All you said are right.
I think we can easily find the PCI clock speed in OS information 
rather than PCI config space. Only OS can determine the PCI 
clock speed (Of course, if system board supports variable clock).

LECOS.
             

 +++ LECOS (Lee, Cheon-Su)   E-mail: lecos@samsung.co.kr
 +-+ Engineer / SOC Development / Samsung Semiconductors
 +++ San#24 Nongseo-Ri,Kiheung-Eup,Yongin-City,Kyunggi-Do,Korea