spi: pxa2xx: Avoid touching SSCR0_SSE on MMP2

spi: pxa2xx: Avoid touching SSCR0_SSE on MMP2

A read from a Winbond W25Q32FV SPI NOR memory chip on my MMP2 returns
wrong data.

It seems like SSE doesn't do the right thing on MMP2 at all. After
enabling the SPI port back again, the FIFO reads return garbage. Things
can be brought back to order by telling the PMU to reset the block.

Here's a good transaction with said chip:

  # busybox devmem 0xd4035000 32 0x00001987 # SSCR0
  # echo 0 >/sys/class/gpio/gpio46/value    # (assert CS)
  # busybox devmem 0xd4035010 32 0x0000009f # SSDR (read ID command)
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff
  # busybox devmem 0xd4035010               # SSDR
  0x000000ef                                # Correct response
  # busybox devmem 0xd4035010               # SSDR
  0x00000040
  # busybox devmem 0xd4035010               # SSDR
  0x00000016
  # busybox devmem 0xd4035010               # SSDR
  0x00000000
  # busybox devmem 0xd4035010               # SSDR
  0x00000000
  # busybox devmem 0xd4035010               # SSDR
  0x00000000
  # echo 1 >/sys/class/gpio/gpio46/value # (deassert CS)
  #

Flipping off an on SSE, then running another transaction:

  # busybox devmem 0xd4035000 32 0x00001907 # SSCR0, SSE off
  # busybox devmem 0xd4035000 32 0x00001987 # SSCR0, SSE on
  # echo 0 >/sys/class/gpio/gpio46/value    # (assert CS)
  # busybox devmem 0xd4035010 32 0x0000009f # SSDR (read ID command)
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010 32 0x00000000 # SSDR
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff                                # Garbage!
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff                                # Oh no
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff
  # busybox devmem 0xd4035010               # SSDR
  0x000000ff
  # echo 1 >/sys/class/gpio/gpio46/value # (deassert CS)
  #

Sometimes the response is not just ones, but something that looks like
bits of a response from a previous transaction.

I can't see a fix other than not touching the SSE altogether after the
device is first brought up.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200118094031.327373-1-lkundrak@v3.sk
Signed-off-by: Mark Brown <broonie@kernel.org>