arm erratas (config options)

[Russell King - ARM Linux]

On Fri, Jun 22, 2012 at 12:05:44PM +0200, Alexander Holler wrote:
> E.g. /proc/cpuinfo talks about variant and revision while all the help  
> texts for the erratas are talking about rNpM (which I would translate to  
> "revision" and "patch level", without knowing the real meanings).

rNpM is a hardware thing.  On the test chips which ARM Ltd supply on
their platforms, it's marked on the case.  With SoC vendors, the only
way to know this definitively is to ask them.

> This makes it necessary to translate "variant" to 'r' and "revision" to  
> 'p', which isn't really obvious (because most people would translate  
> "revision" to 'r' and would wonder how to find the value for 'p' in  
> /proc/cpuinfo or somewhere else).

The variant and revision are terms used in the ARM ARM (or was used at
some point) which don't always reflect the rNpM marking on the package.
Sometimes, a hardware change is made which updates the rNpM but doesn't
update the ID registers.  So they can't be relied upon.

> Another source of confusion about which arm erratas should be enabled  
> for a specific processor is that not all the help texts for the erratas  
> are clear about which variants and revisions are effected. E.g. the help  
> texts for erratas 460075 or 458693 are talking about r2p0, but they  
> doesn't mention if older variants (e.g. r1p3) get hit by these erratas  
> too. The help texts for other erratas (e.g. 743622 and 754322) are  
> talking about r2p*, which I would interpret that this errata applies  
> only to variant 2, but I would never be sure (reading only the help 
> text).

You can enable them all, and the kernel will (attempt) to apply those
which are applicable to your CPU based on the ID register and the values
we know for the ID register corresponding to a particular rNpM part.

The reason they are configuration options is so that you can disable
them and remove that code from the kernel if you wish (and you know
they don't apply) or if you have an explicit need to disable them (you
want to run a test case, or you know that the kernel itself can't apply
the work-around because you're running in non-secure mode.)