bcm2835-sdhost: Adding overclocking option

Allow a different clock speed to be substitued for a requested 50MHz.
This option is exposed using the "overclock_50" DT parameter.
Note that the sdhost interface is restricted to integer divisions of
core_freq, and the highest sensible option for a core_freq of 250MHz
is 84 (250/3 = 83.3MHz), the next being 125 (250/2) which is much too
high.

Use at your own risk.

arch/arm/boot/dts/sdhost-overlay.dts

@@ -13,14 +13,15 @@
 			sdhost: sdhost@7e202000 {
 				compatible = "brcm,bcm2835-sdhost";
 				reg = <0x7e202000 0x100>;
+				pinctrl-names = "default";
+				pinctrl-0 = <&sdhost_pins>;
 				interrupts = <2 24>;
 				clocks = <&clk_sdhost>;
 				dmas = <&dma 13>,
 				       <&dma 13>;
 				dma-names = "tx", "rx";
 				brcm,delay-after-stop = <0>;
-				pinctrl-names = "default";
-				pinctrl-0 = <&sdhost_pins>;
+				brcm,overclock-50 = <0>;
 				status = "okay";
 			};
 
@@ -67,6 +68,7 @@
 
 	__overrides__ {
 		delay_after_stop = <&sdhost>,"brcm,delay-after-stop:0";
+		overclock_50     = <&sdhost>,"brcm,overclock-50:0";
 		force_pio        = <&sdhost>,"brcm,force-pio?";
 		sdhost_freq      = <&clk_sdhost>,"clock-frequency:0";
 	};

drivers/mmc/host/bcm2835-sdhost.c

@@ -184,6 +184,7 @@ struct bcm2835_host {
 	int				max_delay;	/* maximum length of time spent waiting */
 	struct timeval			stop_time;	/* when the last stop was issued */
 	u32				delay_after_stop; /* minimum time between stop and subsequent data transfer */
+	u32				overclock_50;	/* frequency to use when 50MHz is requested (in MHz) */
 };
 
 
@@ -1223,6 +1224,10 @@ static irqreturn_t bcm2835_sdhost_thread_irq(int irq, void *dev_id)
 void bcm2835_sdhost_set_clock(struct bcm2835_host *host, unsigned int clock)
 {
 	int div = 0; /* Initialized for compiler warning */
+	unsigned int input_clock = clock;
+
+	if (host->overclock_50 && (clock == 50000000))
+		clock = host->overclock_50 * 1000000;
 
 	/* The SDCDIV register has 11 bits, and holds (div - 2).
 	   But in data mode the max is 50MHz wihout a minimum, and only the
@@ -1266,13 +1271,18 @@ void bcm2835_sdhost_set_clock(struct bcm2835_host *host, unsigned int clock)
 	if (div > SDCDIV_MAX_CDIV)
 	    div = SDCDIV_MAX_CDIV;
 
-	host->mmc->actual_clock = host->max_clk / (div + 2);
+	clock = host->max_clk / (div + 2);
+	host->mmc->actual_clock = clock;
+
+	if (clock > input_clock)
+		pr_warn("%s: Overclocking to %dHz\n",
+			mmc_hostname(host->mmc), clock);
 
 	host->cdiv = div;
 	bcm2835_sdhost_write(host, host->cdiv, SDCDIV);
 
 	pr_debug(DRIVER_NAME ": clock=%d -> max_clk=%d, cdiv=%x (actual clock %d)\n",
-		 clock, host->max_clk, host->cdiv, host->mmc->actual_clock);
+		 input_clock, host->max_clk, host->cdiv, host->mmc->actual_clock);
 }
 
 static void bcm2835_sdhost_request(struct mmc_host *mmc, struct mmc_request *mrq)
@@ -1572,6 +1582,9 @@ static int bcm2835_sdhost_probe(struct platform_device *pdev)
 		of_property_read_u32(node,
 				     "brcm,delay-after-stop",
 				     &host->delay_after_stop);
+		of_property_read_u32(node,
+				     "brcm,overclock-50",
+				     &host->overclock_50);
 		host->allow_dma = ALLOW_DMA &&
 			!of_property_read_bool(node, "brcm,force-pio");
 	}