If UHS speed modes are enabled, a compatible SD card switches down to 1.8V
during enumeration. If after this a software reboot/crash takes place and
on-chip ROM tries to enumerate the SD card, the difference in
IO voltages (host @ 3.3V and card @ 1.8V) may end up damaging the card.
The fix for this is to have support for power cycling the card in
hardware (with a PORz/soft-reset line causing a power cycle of the card).
Because the beaglebone X15 (rev A,B and C), am57xx-evms and am57xx-idks don't
have this capability, disable voltage switching for these boards.
The major effect of this is that the maximum supported speed mode is now
high speed(50 MHz) down from SDR104(200 MHz).
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
During a short period when the bus voltage is switched from 3.3v to 1.8v,
(to enumerate UHS mode), the mmc module is disabled and the mmc IO lines
are kept in a state according to the programmed pad mux pull type.
According to 4.2.4.2 Timing to Switch Signal Voltage in "SD Specifications
Part 1 Physical Layer Specification Version 5.00 February 22, 2016", the
host should hold CLK low for at least 5ms.
In order to keep the card line low during voltage switch, the pad mux of
mmc1_clk line should be configured to pull down.
Add a new pinctrl group for clock line without pullup to be used in boards
where mmc1_clk line is not connected to an external pullup.
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
