linux-kernel - [PATCH v2] mmc: core: Try other signal levels during power up

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Date:	Thu, 24 Apr 2014 14:44:26 -0700
From:	Tim Kryger <tim.kryger@...aro.org>
To:	Chris Ball <chris@...ntf.net>, Ulf Hansson <ulf.hansson@...aro.org>
CC:	Tim Kryger <tim.kryger@...aro.org>,
	Linux MMC <linux-mmc@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [PATCH v2] mmc: core: Try other signal levels during power up

The eMMC signalling voltage is determined by VCCQ which is provided to
the card by the host.  Signalling is not required to begin at 3.3v and,
if the host and card both support a particular VCC/VCCQ combination, it
can be used immediately.

In contrast, SD Cards must begin with 3.3v signalling and may switch to
a lower voltage signalling if instructed to do so in CMD11.  A message
is required to coordinate this operation because the card only receives
a 3.3v VDD and must know when to use the 1.8v produced by its internal
regulator.

It makes sense for the core to begin with 3.3v signalling but when that
can't be set, 1.8v and 1.2v signalling also should be attempted.  This
is especially important when an external regulator with a limited range
is used to supply VCCQ to an eMMC part.

Signed-off-by: Tim Kryger <tim.kryger@...aro.org>
---

Changes since v1:
  - Eliminated temporary err variable and added debug output

 drivers/mmc/core/core.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index acbc3f2..72926a9 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -1533,8 +1533,13 @@ void mmc_power_up(struct mmc_host *host, u32 ocr)
 	host->ios.timing = MMC_TIMING_LEGACY;
 	mmc_set_ios(host);

-	/* Set signal voltage to 3.3V */
-	__mmc_set_signal_voltage(host, MMC_SIGNAL_VOLTAGE_330);
+	/* Try to set signal voltage to 3.3V but fall back to 1.8v or 1.2v */
+	if (__mmc_set_signal_voltage(host, MMC_SIGNAL_VOLTAGE_330) == 0)
+		dev_dbg(mmc_dev(host), "Initial signal voltage of 3.3v\n");
+	else if (__mmc_set_signal_voltage(host, MMC_SIGNAL_VOLTAGE_180) == 0)
+		dev_dbg(mmc_dev(host), "Initial signal voltage of 1.8v\n");
+	else if (__mmc_set_signal_voltage(host, MMC_SIGNAL_VOLTAGE_120) == 0)
+		dev_dbg(mmc_dev(host), "Initial signal voltage of 1.2v\n");

 	/*
 	 * This delay should be sufficient to allow the power supply
-- 
1.7.9.5

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