The i.MX 8M Plus is a whole different chip than the i.MX 8M Quad. It depends on what you want to do with Librem 5 whether you think that Fir will be better or worse than Evergreen.
- Shrinks from 28nm HPC to 14nm LPC FinFET process node, so more energy efficient
- Shrinks from a 17x17 mm package to a 15x15 mm package, so saves space
- Increases max CPU clock from 1.5 to 1.8 GHz
- Adds a neural processing unit (2.3 TOPS)
- Adds a dual camera image signal processor (2x HC/1x 12 MP) with HDR and dewarp
- Improves the Cortex-M7 subprocessor from 266 to 800 MHz and increased its tightly coupled SRAM from 256 to 512 KB.
- Adds video hardware encoding (H.264 and H.265 at 1080p60), whereas the i.MX 8M Quad has to do software encoding
- Adds a HiFi 4 DSP for audio and natural language processing
- Adds Immersiv3D audio with Dolby Atmos® and DTS:X®
- Increases promise to produce the chip from 10 years to 15 years
- Improves things that aren’t used by the Librem 5 (increases number of USB interfaces from 2 to 3, increases number of Ethernet interfaces from 1 to 2, increases PCIe from version 2.0 to 3.0, etc.)
- Improves the PMIC (hopefully for better power efficiency)
- I think that it increases the number of video interfaces from 2 to 3 (so potentially could have dual screen video out if Purism decides to implement it, but I haven’t read the docs to know how this works)
- Decreases the L2 cache from 1MB to 512KB.
- Reduces the GPU from GC7000Lite (4 shaders) to GC7000UltraLite (2 shaders)
- Decreases the max video out from 60 to 30 frames per second and gets rid of HDR (4Kp60 HDR -> 4Kp30)
- Decreases max video hardware decoding from 4Kp60 HDR to 1080p60 (no HDR)
The i.MX 8M Plus looks a lot better, until you dig into the details. Its image signal processor only supports a max resolution of 12 MP, which isn’t enough to support the 13 MP back camera in the Librem 5, so it probably won’t be used by the back camera, but the front camera might use it. We probably won’t have any software that will take advantage of the neural processing unit and the audio digital signal processor for quite a while. I doubt that the 3D audio in the i.MX 8M Plus will be used, since the Librem 5 already has an advanced audio chip. The Cortex-M core might be great as a processor in low-power mode, but I doubt that will be supported any time soon.
The real advantages I see are that the CPU in Fir will be 20% faster and it will consume less energy than in Evergreen. Also video hardware encoding will make Fir a lot more energy efficient and cooler when shooting video (but it will only be 1080p max resolution video).
However, the tradeoff is that the GPU in fir will be less powerful and the smaller L2 cache probably will hurt performance to some degree. If you play 3D games or want to use the Librem 5 as a PC, you probably are going to prefer Evergreen over Fir. In theory the hardware in Evergreen will support 4Kp video out so it can take advantage of the 4K video decoding whereas Fir is limited to 1080p hardware video decoding. Of course, we don’t know the thermal limits or what will be supported in Evergreen, so its better GPU and better video out might be meaningless in the real world.
Over the long term, we might get the software to take advantage of the NPU and DSP and the better Cortex-M core in fir and it is awesome that NXP is guaranteeing 15 years of production, because that means 15 years of Linux driver updates from NXP. Fir will probably be supported for longer than any phone in the world, but I do have to wonder if anyone will want to carry around the clunky thing 15 years from now.