Hi,
hmm, no big clue here. The flash chip doesn't react at all at lower speeds. I remember a similar case that was reported lately. IIRC, it came down to a floating /HOLD pin. Although your chip doesn't have such a pin, it has two I/O pins (3 and 7) that are not used for the regular SPI interface. You should make sure they have a defined state (e.g. connect them through series resistors to GND or VCC). If you have a multimeter at hand, you can measure the resistance between the pins and GND / VCC each while everything is turned off, and ofc. to which pins on the header they are connected.
I already tried putting the other pins into a defined state, which did not help. But your suggestion about the floating pin put me on the right track: I remembered I saw a hint on the Wiki about ISP and powering the board using the PSU [1] instead of the Raspi which actually did the trick! It now says:
Found Macronix flash chip "MX25L12805D" (16384 kB, SPI) on linux_spi. Found Macronix flash chip "MX25L12835F/MX25L12845E/MX25L12865E" (16384 kB, SPI) on linux_spi. Multiple flash chip definitions match the detected chip(s): "MX25L12805D", "MX25L12835F/MX25L12845E/MX25L12865E" Please specify which chip definition to use with the -c <chipname> option.
(You find a full log attached)
I will do some more experiments when I have some time and maybe add some patches for this chip.
Thanks again for your help!
Fabian