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Date: Wed, 29 Oct 2014 23:36:55 -0700
From: Scott Branden <sbranden@...adcom.com>
To: Ulf Hansson <ulf.hansson@...aro.org>,
Russell King <rmk+kernel@....linux.org.uk>,
Peter Griffin <peter.griffin@...aro.org>,
Stephen Warren <swarren@...dotorg.org>,
Chris Ball <chris@...ntf.net>,
Piotr Krol <pietrushnic@...il.com>
CC: <linux-mmc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
Joe Perches <joe@...ches.com>,
<linux-rpi-kernel@...ts.infradead.org>,
Ray Jui <rjui@...adcom.com>,
<bcm-kernel-feedback-list@...adcom.com>,
Scott Branden <sbranden@...adcom.com>
Subject: [PATCHv2 3/5] mmc: shdci-bcm2835: add efficient back-to-back write workaround
The bcm2835 has clock domain issues when back to back writes to certain
registers are written. The existing driver works around this issue with
udelay. A more efficient method is to store the 8 and 16 bit writes
to the registers affected and then write them as 32 bits at the appropriate
time.
Signed-off-by: Scott Branden <sbranden@...adcom.com>
---
drivers/mmc/host/sdhci-bcm2835.c | 103 ++++++++++++++++++++------------------
1 file changed, 55 insertions(+), 48 deletions(-)
diff --git a/drivers/mmc/host/sdhci-bcm2835.c b/drivers/mmc/host/sdhci-bcm2835.c
index b6cb365..f8c450a 100644
--- a/drivers/mmc/host/sdhci-bcm2835.c
+++ b/drivers/mmc/host/sdhci-bcm2835.c
@@ -24,34 +24,9 @@
#include <linux/mmc/host.h>
#include "sdhci-pltfm.h"
-/*
- * 400KHz is max freq for card ID etc. Use that as min card clock. We need to
- * know the min to enable static calculation of max BCM2835_SDHCI_WRITE_DELAY.
- */
-#define MIN_FREQ 400000
-
-/*
- * The Arasan has a bugette whereby it may lose the content of successive
- * writes to registers that are within two SD-card clock cycles of each other
- * (a clock domain crossing problem). It seems, however, that the data
- * register does not have this problem, which is just as well - otherwise we'd
- * have to nobble the DMA engine too.
- *
- * This should probably be dynamically calculated based on the actual card
- * frequency. However, this is the longest we'll have to wait, and doesn't
- * seem to slow access down too much, so the added complexity doesn't seem
- * worth it for now.
- *
- * 1/MIN_FREQ is (max) time per tick of eMMC clock.
- * 2/MIN_FREQ is time for two ticks.
- * Multiply by 1000000 to get uS per two ticks.
- * *1000000 for uSecs.
- * +1 for hack rounding.
- */
-#define BCM2835_SDHCI_WRITE_DELAY (((2 * 1000000) / MIN_FREQ) + 1)
-
-struct bcm2835_sdhci {
- u32 shadow;
+struct bcm2835_sdhci_host {
+ u32 shadow_cmd;
+ u32 shadow_blk;
};
#define REG_OFFSET_IN_BITS(reg) ((reg) << 3 & 0x18)
@@ -80,33 +55,71 @@ static u8 bcm2835_sdhci_readb(struct sdhci_host *host, int reg)
return byte;
}
-static void bcm2835_sdhci_writel(struct sdhci_host *host, u32 val, int reg)
+static inline void bcm2835_sdhci_writel(struct sdhci_host *host,
+ u32 val, int reg)
{
writel(val, host->ioaddr + reg);
-
- udelay(BCM2835_SDHCI_WRITE_DELAY);
}
-
+/*
+ * The Arasan has a bugette whereby it may lose the content of successive
+ * writes to the same register that are within two SD-card clock cycles of
+ * each other (a clock domain crossing problem). The data
+ * register does not have this problem, which is just as well - otherwise we'd
+ * have to nobble the DMA engine too.
+ *
+ * This wouldn't be a problem with the code except that we can only write the
+ * controller with 32-bit writes. So two different 16-bit registers are
+ * written back to back creates the problem.
+ *
+ * In reality, this only happens when SDHCI_BLOCK_SIZE and SDHCI_BLOCK_COUNT
+ * are written followed by SDHCI_TRANSFER_MODE and SDHCI_COMMAND.
+ * The BLOCK_SIZE and BLOCK_COUNT are meaningless until a command issued so
+ * the work around can be further optimized. We can keep shadow values of
+ * BLOCK_SIZE, BLOCK_COUNT, and TRANSFER_MODE until a COMMAND is issued.
+ * Then, write the BLOCK_SIZE+BLOCK_COUNT in a single 32-bit write followed
+ * by the TRANSFER+COMMAND in another 32-bit write.
+ */
static void bcm2835_sdhci_writew(struct sdhci_host *host, u16 val, int reg)
{
struct sdhci_pltfm_host *pltfm_host = sdhci_priv(host);
- struct bcm2835_sdhci *bcm2835_host = pltfm_host->priv;
- u32 oldval = (reg == SDHCI_COMMAND) ? bcm2835_host->shadow :
- bcm2835_sdhci_readl(host, reg & ~3);
+ struct bcm2835_sdhci_host *bcm2835_host = pltfm_host->priv;
u32 word_shift = REG_OFFSET_IN_BITS(reg);
u32 mask = 0xffff << word_shift;
- u32 newval = (oldval & ~mask) | (val << word_shift);
-
- if (reg == SDHCI_TRANSFER_MODE)
- bcm2835_host->shadow = newval;
- else
+ u32 oldval, newval;
+
+ if (reg == SDHCI_COMMAND) {
+ /* Write the block now as we are issuing a command */
+ if (bcm2835_host->shadow_blk != 0) {
+ bcm2835_sdhci_writel(host, bcm2835_host->shadow_blk,
+ SDHCI_BLOCK_SIZE);
+ bcm2835_host->shadow_blk = 0;
+ }
+ oldval = bcm2835_host->shadow_cmd;
+ } else if (reg == SDHCI_BLOCK_SIZE || reg == SDHCI_BLOCK_COUNT) {
+ /* Block size and count are stored in shadow reg */
+ oldval = bcm2835_host->shadow_blk;
+ } else {
+ /* Read reg, all other registers are not shadowed */
+ oldval = readl(host->ioaddr + (reg & ~3));
+ }
+ newval = (oldval & ~mask) | (val << word_shift);
+
+ if (reg == SDHCI_TRANSFER_MODE) {
+ /* Save the transfer mode until the command is issued */
+ bcm2835_host->shadow_cmd = newval;
+ } else if (reg == SDHCI_BLOCK_SIZE || reg == SDHCI_BLOCK_COUNT) {
+ /* Save the block info until the command is issued */
+ bcm2835_host->shadow_blk = newval;
+ } else {
+ /* Command or other regular 32-bit write */
bcm2835_sdhci_writel(host, newval, reg & ~3);
+ }
}
static void bcm2835_sdhci_writeb(struct sdhci_host *host, u8 val, int reg)
{
- u32 oldval = bcm2835_sdhci_readl(host, reg & ~3);
+ u32 oldval = readl(host->ioaddr + (reg & ~3));
u32 byte_shift = REG_OFFSET_IN_BITS(reg);
u32 mask = 0xff << byte_shift;
u32 newval = (oldval & ~mask) | (val << byte_shift);
@@ -114,11 +127,6 @@ static void bcm2835_sdhci_writeb(struct sdhci_host *host, u8 val, int reg)
bcm2835_sdhci_writel(host, newval, reg & ~3);
}
-static unsigned int bcm2835_sdhci_get_min_clock(struct sdhci_host *host)
-{
- return MIN_FREQ;
-}
-
static const struct sdhci_ops bcm2835_sdhci_ops = {
.read_l = bcm2835_sdhci_readl,
.read_w = bcm2835_sdhci_readw,
@@ -128,7 +136,6 @@ static const struct sdhci_ops bcm2835_sdhci_ops = {
.write_b = bcm2835_sdhci_writeb,
.set_clock = sdhci_set_clock,
.get_max_clock = sdhci_pltfm_clk_get_max_clock,
- .get_min_clock = bcm2835_sdhci_get_min_clock,
.set_bus_width = sdhci_set_bus_width,
.reset = sdhci_reset,
.set_uhs_signaling = sdhci_set_uhs_signaling,
@@ -143,7 +150,7 @@ static const struct sdhci_pltfm_data bcm2835_sdhci_pdata = {
static int bcm2835_sdhci_probe(struct platform_device *pdev)
{
struct sdhci_host *host;
- struct bcm2835_sdhci *bcm2835_host;
+ struct bcm2835_sdhci_host *bcm2835_host;
struct sdhci_pltfm_host *pltfm_host;
int ret;
--
1.7.9.5
--
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