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imx8/usdhc: implement highspeed mode (50Mhz)

--- a/sys/src/9/imx8/usdhc.c

+++ b/sys/src/9/imx8/usdhc.c

@@ -10,6 +10,7 @@

 enum {

 	Initfreq	= 400000,	/* initialisation frequency for MMC */

 	SDfreq		= 25*Mhz,	/* standard SD frequency */

+	SDfreqhs	= 50*Mhz,	/* highspeed frequency */

 	DTO		= 14,		/* data timeout exponent (guesswork) */

 	GoIdle		= 0,		/* mmc/sdio go idle state */

@@ -446,6 +447,15 @@

 				WR(Control0, (RR(Control0) & ~DwidthMask) | Dwidth4);

 				break;

 			}

+		} else {

+			/*

+			 * If card switched into high speed mode, increase clock speed

+			 */

+			if((arg&0x8000000F) == 0x80000001){

+				delay(1);

+				usdhcclk(SDfreqhs);

+				delay(1);

+			}

 		}

 	}else if(cmd == IORWdirect && (arg & ~0xFF) == (1<<31|0<<28|7<<9)){

 		switch(arg & 0x3){

@@ -518,4 +528,5 @@

 	usdhccmd,

 	usdhciosetup,

 	usdhcio,

+	.highspeed = 1,

 };