mmc: mmc_spi: Add Card Detect comments and fix CD GPIO case

This patch fixes the MMC SPI driver from doing polling card detect when a
CD GPIO that supports interrupts is specified using the gpios DT property.

Without this patch the DT node below results in the following output:

spi_gpio: spi-gpio { /* SD2 @ CN12 */
compatible = "spi-gpio";
#address-cells = <1>;
#size-cells = <0>;
gpio-sck = <&gpio6 16 GPIO_ACTIVE_HIGH>;
gpio-mosi = <&gpio6 17 GPIO_ACTIVE_HIGH>;
gpio-miso = <&gpio6 18 GPIO_ACTIVE_HIGH>;
num-chipselects = <1>;
cs-gpios = <&gpio6 21 GPIO_ACTIVE_LOW>;
status = "okay";

spi@0 {
compatible = "mmc-spi-slot";
reg = <0>;
voltage-ranges = <3200 3400>;
spi-max-frequency = <25000000>;
gpios = <&gpio6 22 GPIO_ACTIVE_LOW>; /* CD */
};
};

# dmesg | grep mmc
mmc_spi spi32766.0: SD/MMC host mmc0, no WP, no poweroff, cd polling
mmc0: host does not support reading read-only switch, assuming write-enable
mmc0: new SDHC card on SPI
mmcblk0: mmc0:0000 SU04G 3.69 GiB
mmcblk0: p1

With this patch applied the "cd polling" portion above disappears.

Signed-off-by: Magnus Damm <[e-mail]>
Cc: [e-mail] # v3.18+
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/mmc/host/mmc_spi.c

···
 					     host->pdata->cd_debounce);
 		if (status != 0)
 			goto fail_add_host;
+
+		/* The platform has a CD GPIO signal that may support
+		 * interrupts, so let mmc_gpiod_request_cd_irq() decide
+		 * if polling is needed or not.
+		 */
+		mmc->caps &= ~MMC_CAP_NEEDS_POLL;
 		mmc_gpiod_request_cd_irq(mmc);
 	}