On 10/28/2016 11:38 AM, york sun wrote:
On 10/26/2016 02:02 PM, york.sun@nxp.com wrote:
I came back from my testing and I have more questions than answers.
For _this_ patch, I proposed to flush cache before disabling them, noting once the dcache is disabled, the staled data in dirty cache is not visible to the core. My argument was if we flush L1/L2, they could end up in L3 (I don't know for sure). If I want to skip flushing L3, I have to fix it.
During this discussion, I thought I made a mistake by flushing L1/L2 by way/set first, then flushing by VA. Actually I didn't. I flushed by VA first.
With my today's test, the baseline (working in the sense of booting Linux) is
PATCH 1/2 armv8: Fix dcache disable function PATCH 2/2 armv8: Fix flush_dcache_all function
With these two patches, I flush the stack up to top of U-Boot by VA, followed by flush by set/way. L3 is not flushed. Then d-cache is disabled. I know this is not a real "flush all" procedure. With this modified procedure, I can continue to boot Linux.
If I revert patch 1, i.e. to disable dcache before flushing, I can see the data is not visible from the core (debug with JTAG tool). My hope was the staled data should be flushed to main memory if flushed by VA. That's not the case. The main memory doesn't have the correct data. So my new question is, why flushing by VA doesn't flush the data to main memory? Do I need to flush the cache while cache is enabled?
Guys,
I think I found the root cause of my data loss.
Current code disables D-cache and MMU before flushing the cache. I think the problem is turning off MMU. MMU should stay on when we flush D-cache. We can turn it off after the flushing. Once I make this change, I can see the correct data in memory after flushing (by VA).
Do you agree we should leave MMU on during flushing?
If you're "flushing" by VA, then I'm not surprised since the MMU is what defines the VA->PA mapping, and perhaps you have some physically tagged caches.
However, I believe U-Boot mainline currently "flushes" by set/way, which I wouldn't expect MMU status to influence at all.