The recent commit 0d84c3e6 ("mmc: core: Convert to
mmc_poll_for_busy() for erase/trim/discard") makes use of the
->card_busy() op for SD cards. This uncovered that the ->card_busy() op
in the Meson SDIO driver was never working right:
while polling the busy status with ->card_busy()
meson_mx_mmc_card_busy() reads only one of the two MESON_MX_SDIO_IRQC
register values 0x1f001f10 or 0x1f003f10. This translates to "three out
of four DAT lines are HIGH" and "all four DAT lines are HIGH", which
is interpreted as "the card is busy".

It turns out that no situation can be observed where all four DAT lines
are LOW, meaning the card is not busy anymore. Upon further research the
3.10 vendor driver for this controller does not implement the
->card_busy() op.

Remove the ->card_busy() op from the meson-mx-sdio driver since it is
not working. At the time of writing this patch it is not clear what's
needed to make the ->card_busy() implementation work with this specific
controller hardware. For all use-cases which have previously worked the
MMC_CAP_WAIT_WHILE_BUSY flag is now taking over, even if we don't have
a ->card_busy() op anymore.

Fixes: ed80a13b ("mmc: meson-mx-sdio: Add a driver for the Amlogic Meson8 and Meson8b SoCs")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200416183513.993763-3-martin.blumenstingl@googlemail.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>