Historically for Rockchip devices we've relied on the power-on
default (or perhaps the firmware setting) to get the correct drive
phase for dw_mmc devices. This worked OK for the most part, but:
* Relying on the setting just "being right" is a bit fragile.
* As soon as there is an instance where the power on default is wrong or
where the firmware didn't configure this properly then we'll get a
mysterious failure.
Let's explicitly set this phase in the kernel.
The comments inside this patch try to explain the situation quite
throughly, but the high level overview of this is:
Before this patch on rk3288 devices tested:
* eMMC: 180 degrees
* SDMMC/SDIO0/SDIO1: 90 degrees
After this patch:
* Use 90 degree phase offset usually.
* Use 180 degree phase offset for MMC_DDR52, SDR104, HS200.
That means we are _changing_ behavior for those devices in this way:
* If we have HS200 eMMC or DDR52 eMMC, we'll run ID mode at 90
degrees (vs 180) but otherwise have no change.
* For any non-HS200 / non-DDR52 eMMC devices we'll now _always_ run at
90 degrees (vs 180). It seems fairly unlikely that building modern
hardware is using an eMMC that isn't using DDR52 or HS200, of course.
* For SDR104 cards we'll now run with 180 degree phase offset (vs 90).
It's expected that 90 degree phase offset would have worked OK, but
this gives us extra margin.
I have tested this by inserting my collection of uSD cards (mostly UHS,
though a few not) into a veyron_minnie and confirmed that they still
seem to enumerate properly. For a subset of them I tried putting a
filesystem on them and also tried running mmc_test.
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---
Changes in v2:
- Now use 90 degrees for some modes; updated comments to say why.
drivers/mmc/host/dw_mmc-rockchip.c | 64 ++++++++++++++++++++++++++++++++++++++
1 file changed, 64 insertions(+)
diff --git a/drivers/mmc/host/dw_mmc-rockchip.c b/drivers/mmc/host/dw_mmc-rockchip.c
index 8c20b81cafd8..8068fa887db8 100644
--- a/drivers/mmc/host/dw_mmc-rockchip.c
+++ b/drivers/mmc/host/dw_mmc-rockchip.c
@@ -66,6 +66,70 @@ static void dw_mci_rk3288_set_ios(struct dw_mci *host, struct mmc_ios *ios)
/* Make sure we use phases which we can enumerate with */
if (!IS_ERR(priv->sample_clk))
clk_set_phase(priv->sample_clk, priv->default_sample_phase);
+
+ /*
+ * Set the drive phase offset based on speed mode to achieve hold times.
+ *
+ * That this is _not_ a value that is dynamically tuned and is also
+ * _not_ a value that will vary from board to board. It is a value
+ * that could vary between different SoC models if they had massively
+ * different output clock delays inside their dw_mmc IP block (delay_o),
+ * but since it's OK to overshoot a little we don't need to do complex
+ * calculations and can pick values that will just work for everyone.
+ *
+ * When picking values we'll stick with picking 0/90/180/270 since
+ * those can be made very accurately on all known Rockchip SoCs.
+ *
+ * Note that these values match values from the DesignWare Databook
+ * tables for the most part except for SDR12 and "ID mode". For those
+ * two modes the databook calculations assume a clock in of 50MHz. As
+ * seen above, we always use a clock in rate that is exactly the
+ * card's input clock (times RK3288_CLKGEN_DIV, but that gets divided
+ * back out before the controller sees it).
+ *
+ * From measurement of a single device, it appears that delay_o is
+ * about .5 ns. Since we try to leave a bit of margin, it's expected
+ * that numbers here will be fine even with much larger delay_o
+ * (the 1.4 ns assumed by the DesignWare Databook would result in the
+ * same results, for instance).
+ */
+ if (!IS_ERR(priv->drv_clk)) {
+ int phase;
+
+ /*
+ * In almost all cases a 90 degree phase offset will provide
+ * sufficient hold times across all valid input clock rates
+ * assuming delay_o is not absurd for a given SoC. We'll use
+ * that as a default.
+ */
+ phase = 90;
+
+ switch (ios->timing) {
+ case MMC_TIMING_MMC_DDR52:
+ /*
+ * Since clock in rate with MMC_DDR52 is doubled when
+ * bus width is 8 we need to double the phase offset
+ * to get the same timings.
+ */
+ if (ios->bus_width == MMC_BUS_WIDTH_8)
+ phase = 180;
+ break;
+ case MMC_TIMING_UHS_SDR104:
+ case MMC_TIMING_MMC_HS200:
+ /*
+ * In the case of 150 MHz clock (typical max for
+ * Rockchip SoCs), 90 degree offset will add a delay
+ * of 1.67 ns. That will meet min hold time of .8 ns
+ * as long as clock output delay is < .87 ns. On
+ * SoCs measured this seems to be OK, but it doesn't
+ * hurt to give margin here, so we use 180.
+ */
+ phase = 180;
+ break;
+ }
+
+ clk_set_phase(priv->drv_clk, phase);
+ }
}
#define NUM_PHASES 360
--
2.8.0.rc3.226.g39d4020
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