linux-kernel - [PATCH] MMC Core: Drop initialization frequency floor to 50kHz

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Date:	Thu, 02 Jul 2009 09:49:56 +1000
From:	Ben Nizette <bn@...sdigital.com>
To:	pierre@...man.eu
Cc:	s.hauer@...gutronix.de,
	linux-kernel <linux-kernel@...r.kernel.org>,
	kernel <kernel@...32linux.org>, Ben Nizette <bn@...sdigital.com>
Subject: [PATCH] MMC Core: Drop initialization frequency floor to 50kHz

Patch

commit 8dfd0374be84793360db7fff2e635d2cd3bbcb21
Author: Sascha Hauer <s.hauer@...gutronix.de>
Date:   Thu Apr 9 08:32:02 2009 +0200

    MMC core: limit minimum initialization frequency to 400kHz

Was recently merged.  This is too fast for at least one setup
permutation - the one on my desk which through trial and error won't
initialise at anything above ~350kHz (older Sandisk 256MB SD on
atmel-mci).

To avoid a string of "just found card X which requires clock
(current_clock - epsilon)" this patch drops the floor right down to
50kHz.  This is about the slowest rate before which the discovery
process takes a noticeable slowdown.

Signed-off-by: Ben Nizette <bn@...sdigital.com>
---
diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index d84c880..6ee1931 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -708,12 +708,13 @@ static void mmc_power_up(struct mmc_host *host)
 	 */
 	mmc_delay(10);
 
-	if (host->f_min > 400000) {
-		pr_warning("%s: Minimum clock frequency too high for "
-				"identification mode\n", mmc_hostname(host));
-		host->ios.clock = host->f_min;
-	} else
-		host->ios.clock = 400000;
+	/*
+	 * Card discovery is typically done at the controller's minimum
+	 * allowable frequency but for some controllers this is minimum
+	 * is unreasonably slow.  In that case we limit slow clock rate
+	 * to 50KHz.
+	 */
+	host->ios.clock = max(host->f_min, 50000);
 
 	host->ios.power_mode = MMC_POWER_ON;
 	mmc_set_ios(host);


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