Bug in p1_p2_rdb_pc? Caching-inhibited bit for initial L2 SRAM entry in TLB

[Sinan Akman]

   Hi Pali

On 2022-05-08 11:08 a.m., Pali Rohár wrote:
> On Thursday 14 April 2022 23:05:39 Pali Rohár wrote:
>> + Sinan
>>
>> On Wednesday 13 April 2022 11:26:33 Pali Rohár wrote:
>>> On Tuesday 05 April 2022 10:57:37 Pali Rohár wrote:
>>>> Hello!
>>>>
>>>> I suspect that there is a bug in board/freescale/p1_p2_rdb_pc/tlb.c code
>>>> which configures TLB entry for initial L2 SRAM.
>>>>
>>>> When L2 is 512 kB long (e.g. on P2020) then U-Boot *unsets* MAS2_I bit
>>>> for first half of L2 and for second half of L2 U-Boot *sets* this bit.
>>>>
>>>> See code:
>>>> https://source.denx.de/u-boot/u-boot/-/blob/v2022.04/board/freescale/p1_p2_rdb_pc/tlb.c#L99-104
>>>>
>>>> I do not think that one part of L2 SRAM should be configured differently
>>>> as second part. Therefore I think that this is a bug in U-Boot code.
>>>>
>>>> Do you know is correct configuration of TLB entries for initial L2 SRAM?
>>>>
>>>> MAS2_I is Caching-inhibited bit which is described as:
>>>>
>>>> Caching-inhibited:
>>>> * 0 - Accesses to this page are considered cacheable.
>>>> * 1 - The page is considered caching-inhibited. All loads and stores to
>>>>        the page bypass the caches and are performed directly to main
>>>>        memory. A read or write to a caching-inhibited page affects only
>>>>        the memory element specified by the operation.
>>> Hello! I found EREF: A Programmer’s Reference Manual for Freescale Power
>>> Architecture Processors Supports e500 core family (e500v1, e500v2,
>>> e500mc, e5500, e6500) e200 core family document at NXP web:
>>>
>>> https://www.nxp.com/files-static/32bit/doc/ref_manual/EREF_RM.pdf
>>>
>>> And section "Cache and MMU Architecture" in part 7.3.1.2.2 Unable to
>>> Lock Conditions (page 763) contains following information:
>>>
>>> If no exceptions occur and no overlocking condition exists, an attempt
>>> to set a lock can fail if any of the following is true:
>>>
>>> • The target address is marked cache-inhibited, or the storage
>>>    attributes of the address uses a coherency protocol that does not
>>>    support locking
>>>
>>> So for me it looks like that L2 SRAM (which works at L2 with locked
>>> cache lines) should not set MA2_I (cache-inhibited) bit.
>>>
>>> Any opinion? Or you do have some more information?
> Hello!
>
> I looked at it again and it is more complicated as I initially thought.
>
> There are two options how L2 cache on P2020 may be used as SRAM. First
> option is the classic way with locking lines, like it is done on other
> architectures. Second option seems to be P1/P2 specific as it is *not*
> documented in e500 core reference manual, but in P2020 SoC reference
> manual, and this option changes L2 operation from Cache to Memory-Mapped
> SRAM mode.
>
> Downloading P2020 reference manual (rev2) requires NXP account:
> https://www.nxp.com/webapp/Download?colCode=P2020RM
>
> But some older version (rev0) can be found on internet, e.g.:
> http://m4udit.dinauz.org/P2020RM_rev0.pdf
>
> I checked U-Boot code and it is for L2 SRAM configuration uses second
> option, therefore not via L2 locked lines, but as L2 memory-mapping.
>
> P2020 reference manual in section "Memory-Mapped SRAM Coherency Rules"
> contains:
>
>    Accesses to memory-mapped SRAM are cacheable only in the corresponding
>    e500 L1 caches. External accesses must be marked cache-inhibited or be
>    performed with non-caching transactions.
>
> So based on the fact that L2 is in U-Boot in memory-mapped SRAM mode,
> not in cache mode with locked lines and that P2020 RM explicitly says
> that memory-mapped SRAM can be cacheable in L1, my understanding is that
> TLB mapping for L2 SRAM should work with Caching-inhibited bit set and
> also with unset (when unset then caching is disabled at L1 level).
>
> Is my deduction correct?
>
> Priyanka, Sinan, any idea?

   I am still away from my boards but in few weeks I will
take a deeper look at this. I need to run some test to see
and understand how this is all implemented.

   Thanks for all the work you do for P2020.

   Regards
   Sinan Akman