Re: [PATCH] Enable AT_PLATFORM for Loongson 2F CPU

[David Daney <ddaney@caviumnetworks.com>]

On 11/08/2010 02:27 PM, Robert Millan wrote:
> 2010/11/8 David Daney<ddaney@caviumnetworks.com>:
> > I seems weird to me that you cannot get this information from the PRId
> > register.  Perhaps the documentation is defective.
> Yes, I agree it's really shortsighted of them, but I don't
> see any way around it.
>
> > The Chinese version of the Loongson2E user guide seems to say something
> > about the two lower nibbles of the PRId, but being a non-chinese reader, I
> > have no idea if it would be relevant.
> >
> > I would think that the low order bits of the register can reliably
> > differentiate these two parts.
> There are English versions for both 2E and 2F.  See page 66
> of the Loongson 2F document:
>
> http://dev.lemote.com/files/resource/documents/Loongson/ls2f/Loongson2FUserGuide.pdf
>
> <quote>
> The revision number can distinguish some chip revisions, however there
> is no guarantee
> that changes to the chip will necessarily be reflected in the PRId
> register, or that changes to
> the revision number necessarily reflect real chip changes. For this
> reason, software should
> not rely on the revision number in the PRId register to characterize the chip.
> </quote>
>
> Page 72 of the Loongson 2E document
> (http://www.lemote.com/upfiles/godson2e-user-manual-V0.6.pdf) has the
> same text.
>
> In both documents, the lower byte is defined as "Revision number",
> and its value is 0x02 (for both 2E and 2F).
>
> If you'd rather not assume the docs are correct, I can test if
> my Yeeloong (Loongson 2F) has 0x02, but then in case it's
> something higher, would you be willing to assume:
>
>    rev<= 0x02  -->   2E
>    rev>  0x02  -->  2F
No, I refuse to assume that.


Look at the description of the register in this document:

http://dev.lemote.com/files/resource/documents/Loongson/ls2e/godson2e.user.manual.pdf

On page 49, it says that bits 0-3 contain the minor version number and 
bits 4-7 are the major version number (according to a workmate that 
reads Chinese).
I don't know for certain, but it seems plausible that the 2E and 2F will 
differ in bits 4-7, and bits 0-3 can be ignored.


> or similar logic?  This seems risky if we take into account
> that there's no guarantee from the vendor.