Re: [PATCH 9/9] [DNI] ARM: multi_v7_defconfig: Enable CONFIG_ARM_LPAE for multi_v7_config

On Thu, Jan 19, 2023, at 16:27, Alexander Stein wrote:

Am Donnerstag, 19. Januar 2023, 16:09:05 CET schrieb Arnd Bergmann:

quoted

On Thu, Jan 19, 2023, at 15:42, Alexander Stein wrote:

quoted

This is necessary to support PCIe on LS1021A.

Signed-off-by: Alexander Stein <redacted>

Can you explain why this is actually required? I can see that the
ranges in the PCIe device point to a high address (0x4000000000,
2^40), but I can't tell if this is hardwired in the SoC or a
setting that is applied by software (either the bootloader or
the PCIe driver).

The RM ([1]) memory map (Table 2-1) says that 'PCI Express 1' is located at 
'400000_0000', 'PCI Express 2' at '480000_0000', so I assume this is hardcoded 
in SoC.
It also explicitly lists in that table PCIe 1&2 is only accessible with 40-bit 
addressing.

quoted

If you can reprogram the memory map, I would expect this to fit
easily into the 32-bit address space, with 1GB for DDR3 memory
and 1GB for PCIe BARs.

I'm not sure which part of memory map you can reprogram and where, but I guess 
this is fixed on this SoC.

Ok, I see it now. It looks like they fell victim to the 
cursed "Principles of ARM® Memory Maps White Paper"
document and messed it up even further ;-)

In particular, it seems that the memory map of the PCI address
spaces is configurable, but only within that area you listed.
I see that section "28.4.2 PEX register descriptions" does list
a 64-bit prefetchable address space in addition to the 32-bit
non-prefetchable memory space, but the 64-bit space is not
listed in the DT. It would be a good idea to configure that
as well in order for devices to work that need a larger BAR,
such as a GPU, but it wouldn't help with fitting the PCIe
into non-LPAE 32-bit CPU address space.

In the datasheet I also see that the chip theoretically
supports 8GB of DDR4, which would definitely put it beyond
the highmem limit, even with the 4G:4G memory split. Do you
know if there are ls1021a devices with more than 4GB of
installed memory?

    Arnd

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