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

[Stephen Warren]

On 10/18/2016 12:40 PM, york sun wrote:
> 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.

By "hit" here, I mean that even with the dcache disabled, when the CPU 
performs a read access, if the dcache contains a copy of that data, it 
can return it rather than requiring it to be fetched from DRAM.

Yes, with the dcache disabled, I would not expect any writes to allocate 
new lines in the cache (although presumably writes would update any 
lines already there, in a write-though sense).

At least, I'm pretty sure this is all true. It seems the only way to 
allow switching from cache-on to cache-off state without losing dirty data.