Re: Multi-platform, and secure-only ARM errata workarounds

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 03/05/2013 12:40 AM, Peter De Schrijver wrote:
> On Mon, Mar 04, 2013 at 06:08:27PM +0100, Stephen Warren wrote:
>> On 03/04/2013 02:16 AM, Peter De Schrijver wrote:
>>> On Mon, Mar 04, 2013 at 07:34:36AM +0100, Peter De Schrijver wrote:
>>>> On Fri, Mar 01, 2013 at 06:37:27PM +0100, Stephen Warren wrote:
>>>>
> 
> ...
> 
>>> 1) Handle CPU0 errata WARs in the bootloader
>>
>> OK - there's not much choice here, and I've posted a patch for this for
>> Tegra U-Boot already.
>>
>>> 2) Indicate in device tree if linux is booting in secude mode or non-secure
>>>    mode.
>>> 3) Use this information in the kernel to decide how to apply the WARs for
>>>    secondary core bringup and after powerungating.
>>
>> Hmmm. That seems like a lot of overhead to avoid duplicating roughly 8
>> assembly instructions per Tegra version. Also, some/all of the WARs in
> 
> Unfortunately we can't write to the diag register if we are in non-secure
> mode. So unless we never want to support running in non-secure mode, we will
> need to make the distinction somehow and use a different method for non-secure
> mode. Or assume the secure OS has applied the WARs.

Yes. The secure OS really has to have enabled the appropriate WARs
before jumping into the kernel's reset vector. If/when we support the
upstream kernel running on Tegra in non-secure mode, the plan was to use
a Tegra-specific mechanism to detect secure-vs-normal mode in the Tegra
reset vector, and skip the application of secure-only WARs based on that.

> I'm afraid existing secure
> OS implementations for Tegra don't work that way though. They just offer an
> SMC which allows the kernel to read and write the diag register.

I had a downstream discussion about this, and Bo Yan said someone had
verified this was working correctly for at least for some WARs on some
CPUs and for the one particular secure OS we're using.

I think it's reasonable to require a fixed secure OS (i.e. one that
correctly enables any required WARs) be used with any upstream kernel,
since running in normal world would be a new feature that we'd be
supporting.

An SMC to read/write the diag register sounds the opposite of secure...

>> question probably need to be applied very early by assembly code, e.g.
>> before MMU is re-enabled, so I think you'd end up parsing DT from
>> assembly again, which would be painful. I tend to think just including
>> the code in the kernel's SoC-specific reset handler is simplest, and
>> even with the slight duplication, probably most maintainable. I've
>> written a patch for this for Tegra already, which I hope to post later
>> today, depending on testing and what other stuff I get side-tracked on.
> 
> No. We could just set a flag in __tegra_cpu_reset_handler_data based on the
> info in DT or use a different reset handler. DT is parsed before bringing up
> secondary CPUs, so this approach should work I think.

Yes, that could work.