Re: CXL Boot to Bash - Section 2a (Drivers): CXL Decoder Programming

[Zhijian Li (Fujitsu)]

Hi Gregory,


On 07/03/2025 07:56, Gregory Price wrote:
> What if instead, we had two 256MB endpoints on the same host bridge?
> 
> ```
> CEDT
>             Subtable Type : 01 [CXL Fixed Memory Window Structure]
>                  Reserved : 00
>                    Length : 002C
>                  Reserved : 00000000
>       Window base address : 0000000100000000   <- Memory Region
>               Window size : 0000000020000000   <- 512MB
> Interleave Members (2^n) : 00                 <- Not interleaved
> 
> Memory Map:
>    [mem 0x0000000100000000-0x0000000120000000] usable  <- SPA
> 
> Decoders
>                              decoder0.0
>                    range=[0x100000000, 0x120000000]
>                                  |
>                              decoder1.0
>                    range=[0x100000000, 0x120000000]
>                    /                              \
>              decoded2.0                        decoder3.0
>    range=[0x100000000, 0x110000000]   range=[0x110000000, 0x120000000]
> ```

It reminds me that during construct_region(), it requires decoder range in the
switch/host-bridge is exact same with the endpoint decoder. see
match_switch_decoder_by_range()

If so, does following decoders make sense?


  Decoders
                               decoder0.0
                     range=[0x100000000, 0x120000000]
                                   |
                      +------------+-----------+
                     /                          \
                    |      Host-bridge contains  |
              decoder1.0     2 decoders       decoder1.1
      range=[0x100000000, 0x110000000]     range=[0x110000000, 0x120000000]
                     /                              \
               decoded2.0                        decoder3.0
     range=[0x100000000, 0x110000000]   range=[0x110000000, 0x120000000]

Thanks
Zhijian