[U-Boot] [Patch v4 2/5] ARMv8: Adjust MMU setup

[York Sun]

On 06/06/2014 05:33 AM, Mark Rutland wrote:
> [...]
> 
>>>>> What you need to do to replace the active set of tables (assuming that
>>>>> the new mapping has the instruction stream mapped in an identical way)
>>>>> is:
>>>>>
>>>>> - Write the tables.
>>>>>
>>>>> - DSB to make them visible to the MMU.
>>>>>
>>>>> - Write to the appropriate TTBR_*.
>>>>>
>>>>> - ISB to complete the write to the TTBR_*.
>>>>>
>>>>> - TLBI to invalidate the old mappings the the TLBs.
>>>>>
>>>>> - DSB to complete the TLBI.
>>>>>
>>>>> - If you've changed the instruction stream or system state that will
>>>>>   affect the instruction stream, ISB to flush the CPU pipeline.
>>>>>
>>>>>
>>>> Here is the flow I have (as of v5 patch)
>>>>
>>>> Write the tables
>>>>
>>>> (I removed dsb here in v5, need to add back)
>>>>
>>>> Write TTBR
>>>>
>>>> (I missed isb here, need to add)
>>>>
>>>> Flush dcache (otherwise the table will not be in DDR. Yes, I verified)
>>>
>>> This looks odd -- why do we need the tables to be in DDR? Why would we
>>> flush them here, when the address is partially visible to the MMU?
>>
>> This sounds odd but it actually makes sense. Let's say we have new tables
>> created by u-boot. The new tables are in the address of DDR with D-cache
>> enabled. If we don't flush the cache, the moment TLBs are invalidated, MMU will
>> fetch the tables from TTBR which points to DDR. Without a valid TLB, MMU cannot
>> get the table from D-cache, it has to fetch from the DDR directly. I have
>> verified this by checking waveforms of the SoC and exercised code in both ways.
> 
> How is TCR_EL2.SH0 (or TCR_EL1.SH*) configured?
> 
> You'll only need to flush the cache if they're configured non shareable.

It is configured as non shareable.

York