mmc: core: fix performance regression initializing MMC host controllers

Commit fa550189 introduced a performance
regression by adding mmc_power_up() to mmc_start_host().  mmc_power_up()
is not necessary to host controller initialization, it is part of card
initialization and is performed anyway asynchronously.

This patch allows a driver to leave the power up in asynchronous code
(as it was before).

On my current target platform this reduces driver initialization from:

[    1.313220] initcall sdhci_acpi_driver_init+0x0/0x12 returned 0 after 102008 usecs

to this:

[    1.217209] initcall sdhci_acpi_driver_init+0x0/0x12 returned 0 after 8331 usecs
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/core/core.c

@@ -2416,7 +2416,8 @@ void mmc_start_host(struct mmc_host *host)
 {
 	host->f_init = max(freqs[0], host->f_min);
 	host->rescan_disable = 0;
-	mmc_power_up(host);
+	if (!(host->caps2 & MMC_CAP2_NO_PRESCAN_POWERUP))
+		mmc_power_up(host);
 	mmc_detect_change(host, 0);
 }
 

drivers/mmc/host/sdhci-acpi.c

@@ -195,6 +195,8 @@ static int sdhci_acpi_probe(struct platform_device *pdev)
 		host->mmc->pm_caps  |= c->slot->pm_caps;
 	}
 
+	host->mmc->caps2 |= MMC_CAP2_NO_PRESCAN_POWERUP;
+
 	err = sdhci_add_host(host);
 	if (err)
 		goto err_free;

include/linux/mmc/host.h

@@ -280,6 +280,7 @@ struct mmc_host {
 #define MMC_CAP2_PACKED_WR	(1 << 13)	/* Allow packed write */
 #define MMC_CAP2_PACKED_CMD	(MMC_CAP2_PACKED_RD | \
 				 MMC_CAP2_PACKED_WR)
+#define MMC_CAP2_NO_PRESCAN_POWERUP (1 << 14)	/* Don't power up before scan */
 
 	mmc_pm_flag_t		pm_caps;	/* supported pm features */