mmc: dw_mmc: Don't allow Runtime PM for SDIO cards

mmc: dw_mmc: Don't allow Runtime PM for SDIO cards

According to the SDIO standard interrupts are normally signalled in a
very complicated way.  They require the card clock to be running and
require the controller to be paying close attention to the signals
coming from the card.  This simply can't happen with the clock stopped
or with the controller in a low power mode.

To that end, we'll disable runtime_pm when we detect that an SDIO card
was inserted.  This is much like with what we do with the special
"SDMMC_CLKEN_LOW_PWR" bit that dw_mmc supports.

NOTE: we specifically do this Runtime PM disabling at card init time
rather than in the enable_sdio_irq() callback.  This is _different_
than how SDHCI does it.  Why do we do it differently?

- Unlike SDHCI, dw_mmc uses the standard sdio_irq code in Linux (AKA
  dw_mmc doesn't set MMC_CAP2_SDIO_IRQ_NOTHREAD).
- Because we use the standard sdio_irq code:
  - We see a constant stream of enable_sdio_irq(0) and
    enable_sdio_irq(1) calls.  This is because the standard code
    disables interrupts while processing and re-enables them after.
  - While interrupts are disabled, there's technically a period where
    we could get runtime disabled while processing interrupts.
  - If we are runtime disabled while processing interrupts, we'll
    reset the controller at resume time (see dw_mci_runtime_resume),
    which seems like a terrible idea because we could possibly have
    another interrupt pending.

To fix the above isues we'd want to put something in the standard
sdio_irq code that makes sure to call pm_runtime get/put when
interrupts are being actively being processed.  That's possible to do,
but it seems like a more complicated mechanism when we really just
want the runtime pm disabled always for SDIO cards given that all the
other bits needed to get Runtime PM vs. SDIO just aren't there.

NOTE: at some point in time someone might come up with a fancy way to
do SDIO interrupts and still allow (some) amount of runtime PM.
Technically we could turn off the card clock if we used an alternate
way of signaling SDIO interrupts (and out of band interrupt is one way
to do this).  We probably wouldn't actually want to fully runtime
suspend in this case though--at least not with the current
dw_mci_runtime_resume() which basically fully resets the controller at
resume time.

Fixes: e9ed8835e990 ("mmc: dw_mmc: add runtime PM callback")
Cc: <stable@vger.kernel.org>
Reported-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>