[U-Boot] [PATCH 1/2] armv8: add hooks for all cache-wide operations

[york sun]

On 10/18/2016 11:14 AM, Stephen Warren wrote:
> On 10/18/2016 09:28 AM, york sun wrote:
>> On 10/17/2016 04:35 PM, Stephen Warren wrote:
>>> From: Stephen Warren <swarren at nvidia.com>
>>>
>>> SoC-specific logic may be required for all forms of cache-wide
>>> operations; invalidate and flush of both dcache and icache (note that
>>> only 3 of the 4 possible combinations make sense, since the icache never
>>> contains dirty lines). This patch adds an optional hook for all
>>> implemented cache-wide operations, and renames the one existing hook to
>>> better represent exactly which operation it is implementing. A dummy
>>> no-op implementation of each hook is provided. These dummy
>>> implementations are moved into C code, since there's no need to
>>> implement them in assembly.
>>>
>> Stephen,
>>
>> Moving this function to C may pose an issue. I had a debug a couple of
>> years ago that calling a C function put the stack into cache after
>> flushing L1/L2. That's why I used asm function to flush L3.
>
> Assuming the stack is located in cachable memory, the CPU is free (per
> the definition of the ARM architecture) to pull it into the cache at any
> time the cache is enabled (and perhaps even when it isn't enabled, at
> the very least for the icache on ARMv8 if not other cases too).
> Implementation in C vs. assembly has absolutely no effect here. I guess
> your statement assumes that C functions will write data to the stack and
> assembly functions never will. There's no strict 1:1 correlation between
> those two things; assembly code can touch the stack just like C code. If
> there's an assumption it won't, it needs to be documented in the header
> defining these hook functions.
>
> I assume you're specifically talking about dirtying the dcache between
> the point when dcache flushing starts and the point when the dcache is
> disabled? If so, flush_dcache_all() itself would have to be manually
> coded in assembly to avoid using the stack, as would dcache_disable()
> and set_sctlr(). I think this is why dcache_disable() currently disables
> the dcache first (thus preventing it acquiring new dirty data) and then
> flushes the dcache afterwards (thus guaranteeing that all dirty data is
> flushed with no race condition). This implies that your change to swap
> the order of those two functions isn't correct. I'm pretty sure I'm

I wonder if David can shed some light on the original order of calls to 
disable dcache.

> correct in saying that the dcache can hit even if it's disabled, hence
> disabling the dcache while it contains dirty data won't lead to issues?
>

My earlier debug was based on the original order of calls. I found I had 
to avoid using the stack before flushing L3. Now with the changed order, 
I haven't tested. But I can image the stack will be dirty and flushing 
L3 may or may not push the data into main memory (depending on the L3 
implementation whether inclusive or not).

You said you are sure dcache can hit even if it is disabled. Can you 
explain more? My test shows as soon as the d-cache is disabled, the core 
cannot get the data in dirty cache.

York