When backporting Commit e419990b5e8 ("mmc: dw_mmc: correct the
calculation for CLKDIV") to 3.4 kernel and debugging
a FW issue, I found the code unreadable. This rewrite simplifies
the computation and explains each step.
Signed-off-by: Grant Grundler <grundler@xxxxxxxxxxxx>
---
V2: rewrote commit msg per feedback
Tested on Samsung Series 3 Chromebook (exynos 5250 chipset) using
ChromeOS 3.4 kernel (not 3.9-rc3 which this patch is based against).
I've posted the test_dw_mmc.c source to confirm this patch generates
the same correct values.
The CLKDIV issue I was trying to resolve in ChromeOS 3.4 kernel:
http://crbug.com/221828
drivers/mmc/host/dw_mmc.c | 22 +++++++++++++++-------
1 file changed, 15 insertions(+), 7 deletions(-)
diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
index 9834221..6fdf55b 100644
--- a/drivers/mmc/host/dw_mmc.c
+++ b/drivers/mmc/host/dw_mmc.c
@@ -631,14 +631,22 @@ static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit)
if (slot->clock != host->current_speed || force_clkinit) {
div = host->bus_hz / slot->clock;
- if (host->bus_hz % slot->clock && host->bus_hz > slot->clock)
- /*
- * move the + 1 after the divide to prevent
- * over-clocking the card.
+ if (host->bus_hz > slot->clock) {
+ /* don't overclock due to integer math losses */
+ if ((div * slot->clock) < host->bus_hz)
+ div++;
+
+ /* don't overclock due to resolution of HW */
+ if (div & 1)
+ div++;
+
+ /* See 6.2.3 CLKDIV in "Mobile Storage Host Databook"
+ * Look for dwc_mobile_storage_db.pdf from Synopsys.
+ * CLKDIV value 0 means divisor 1, val 1 -> 2, ...
*/
- div += 1;
-
- div = (host->bus_hz != slot->clock) ? DIV_ROUND_UP(div, 2) : 0;
+ div /= 2;
+ } else
+ div = 0; /* use bus_hz */
dev_info(&slot->mmc->class_dev,
"Bus speed (slot %d) = %dHz (slot req %dHz, actual %dHZ"
--
1.8.1.3
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