svn commit: r274739 - head/sys/mips/conf

[Warner Losh]

> On Nov 25, 2014, at 11:42 AM, Dag-Erling Smørgrav <des at des.no> wrote:
> 
> Ian Lepore <ian at FreeBSD.org> writes:
>> The clock all this is being measured with runs at 32 KHz.  That's 'K'.
> 
> That explains a lot.
> 
> Is there absolutely no instruction counter in this chip?  Any other
> devices we can use, such as temperature sensors or ADCs?

There’s no temperature sensors or useful ADCs. Many of the chips
don’t have any ADC. Some have ADCs, but the OS can’t necessarily
use them easily in the boot process. Any ADCs that we might have
on the chip (and it varies from chip to chip which we won’t know until
late in the boot process) aren’t necessarily wired to anything
and often times share pins with other functions which could be quite
disruptive to bring on line (ADC on an unconnected line on these
chips likely isn’t going to give that much entropy since you’re
measuring grounded pins).

There’s really no good sources of entropy in these systems until
unpredictable peripherals get online (network, SD cards, etc) and
then it is too late.

These chips are based on the ARM920 and ARM926 cores, which
don’t have instruction counters, unlike their newer brethren. This is
fairly typical of the embedded space: unless you really need feature
X, it is omitted.

Warner

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