So the TMEM wasn't cache, but manually managed memory split into 8 512 byte banks that had to be loaded from the RDP's command list stream. That's half the problem.
Additionally, the TMEM could only be loaded from RDRAM, not directly from the cartridge. I think the RDP's DMA master is only connected to the RDRAM slave port and not the main system's bus matrix.
So going back to it, games would a lot of the time store compressed data with a simple algorithm that could run out of the CPU's cache. Then the scheme looks like
* Cart->RDRAM DMA of compressed texture
* CPU decompresses texture into another RDRAM bank, and can be considered a RDRAM->RDRAM transfer. Sometimes the RSP handles this instead. I'm not sure if you could load straight out of RSP DMEM to avoid another bounce to RDRAM. I don't think XBUS works that way, but I could be wrong.
* RDRAM->TMEM DMA of uncompressed texture
Interestingly, games with more advanced texturing schemes like Indiana Jones tended to use uncompressed textures. They did this to avoid the decompression step and it's bandwidth. At that point it's just staging the texture with that cart's DMA, and slurping that into TMEM without any other processors eating bandwidth in between.
Additionally, the TMEM could only be loaded from RDRAM, not directly from the cartridge. I think the RDP's DMA master is only connected to the RDRAM slave port and not the main system's bus matrix.
So going back to it, games would a lot of the time store compressed data with a simple algorithm that could run out of the CPU's cache. Then the scheme looks like
* Cart->RDRAM DMA of compressed texture
* CPU decompresses texture into another RDRAM bank, and can be considered a RDRAM->RDRAM transfer. Sometimes the RSP handles this instead. I'm not sure if you could load straight out of RSP DMEM to avoid another bounce to RDRAM. I don't think XBUS works that way, but I could be wrong.
* RDRAM->TMEM DMA of uncompressed texture
Interestingly, games with more advanced texturing schemes like Indiana Jones tended to use uncompressed textures. They did this to avoid the decompression step and it's bandwidth. At that point it's just staging the texture with that cart's DMA, and slurping that into TMEM without any other processors eating bandwidth in between.