Re: 2017-06-20-netbsd-raspi-earmv6hf.img

[John Klos <john%ziaspace.com@localhost>]

> SD High-Speed mode is speed/performance-wise equivalent to UHS SDR25 mode, 
> which runs the bus up to 50MHz and can do up to 25MB/s transfers. The 
> difference between High-Speed and SDR25 is signalling voltage (3.3V vs 1.8V).
>
> UHS SDR50 mode runs the bus up to 100MHz and can do up to 50MB/s transfers. 
> 1.8V signaling is the only option here.

We know that SDR50 mode works with many, perhaps most, cards under NetBSD. 
We can get greater than 25 MB/sec actual transfer on some cards. That 
means that SDR50 (100 MHz clock, 50 MB/sec) is supported in hardware and 
works:

frigg# dd if=/dev/rld0b of=/dev/null bs=4m
256+0 records in
256+0 records out
1073741824 bytes transferred in 40.947 secs (26222722 bytes/sec)

Some cards don't work at SDR50 speeds. We can:

1) Have a boot-time option in cmdline.txt or config.txt which selects a 
slower mode

2) Have the kernel try slower modes if we get "cmd end bit error" events

3) Have all Pis with all fast cards run slower :P

John