The PAN9026 SDIO hardware interface is detected by the SDIO master, which is the host processor platform.
The VIOSD voltage for PAN9026 should be supplied by the VIOSD voltage from host processor. Means, if the master is supplied with VIOSD = 1.8V, you should supply the same voltage to VIOSD of PAN9026.
By using VIOSD = 1.8V the master and PAN9026 will be able to use SDIO 3.0 Card specification with Ultra-High-Speed (UHS) modes up to SDR50 / DDR50.
It is very important, that the Linux System configuration (devicetree) for the peripheral SDIO master device is set to 1V8 voltage use (usually set by default).
Check for example with: "cat /sys/kernel/debug/mmc0/ios"
Example for VIOSD = 3.3V use at our PAN9026-IMX Wandboard Ubuntu platform:
# sudo cat /sys/kernel/debug/mmc0/ios
clock: 50000000 Hz
actual clock: 495000000 Hz
vdd: 21 (3.3 ~ 3.4 V)
bus mode: 2 (push-pull)
chip select: 0 (don't care)
power mode: 2 (on)
bus width: 2 (4 bits)
timing spec: 0 (sd high-speed)
signal voltage: 0 (3.30 V)
driver type: 0 (driver type B)