Unable to read data from flash via Generic Serial Flash Interface (only zeros)

Hello,

in my current Arria 10 GX design I have one small Micron flash for the FPGA binary (MT25QU256ABA1EW9, 32MByte) and two larger Micron flash devices (MT25QU01GBBB1EW9, 128 MByte each) for random data.

I can write, read and erase the smaller flash via the GSFI without any problem.

When reading one of the larger flash I only get back zeros via the Avalon-MM memory slave of the GSFI, but I can clearly see that there is data transferred on the SPI bus when I watch it in SignalTap, as seen below.

Purple box: Command (8 clock cycles, 1 Byte)
Green box: Address (8 clock cycles, 4 Byte)
Yellow box: Dummy cycles (10 clock cycles)
Red box: Readdata from flash (8 clock cycles, 4 Byte) - content: 0x87654321
Red circle: Readdatavalid from GSFI - content: 0x00000000

Unfortunately I can't watch the read transfer from the smaller program flash, because the SPI pins are not exported from the GSFI.

The CSR registers of the GSFI and the flash registers are initialized the following way and identical for the smaller and larger flash devices:

1. Activate 4-Byte addressing in flash:
   1. Write 0x000000B7 to flash command setting register (0x1C)
   2. Write 0x1 to flash command control register (0x20)
2. Write 0x00000101 to control register (0x00)
3. Write 0x1 to baud rate divider register (0x04)
4. Write 0x0 to CS delay setting register (0x08)
5. Write 0x00000AEB (10 dummy cycles & read opcode 0xEB) to read instruction register (0x14)
6. Write 0x00022220 to operating protocols setting register (0x10)

I have seen a similar problem posted on here, but the solution (removing the SFL) doesn't work for me, because I don't instantiate one. (see Solved: Re: Re:Arria 10 can't read from flash via Generic Serial Flash Interface - Intel Communities)

Does anyone have a clue why the data seen on the SPI bus doesn't show up on the Avalon-MM memory slave readdata signal when readdatavalid is high?

Best Regards,
Florian