Hi Minwoo,
On 21-08-2026 14:42, 박민우 (MW Park) wrote:
When U-Boot hands off to the OS, mmc_deinit() renegotiates the bus
down out of HS200/HS400 by issuing CMD6 switches and retuning, then
With HS200/HS400 stripped, the path issues CMD6 to switch to HS
timing and then reselects a slower mode(which doesn't require
tuning). Please drop "retuning" (same in the Kconfig help).
changes the clock. For eMMC this is unnecessary: the OS issues CMD0
(card reset), which returns the device to backward-compatible mode,
CMD0 returns the protocol state machine to Idle. It is not a
hardware card reset (that is RST_n or a power cycle). Might be
worth wording it that way.
and renegotiates from scratch regardless of the mode U-Boot left the
device in.
Please refer to fceea9926804 ("mmc: Downgrade SD/MMC from
UHS/HS200/HS400 modes before boot") for the original motivation.
Add a Kconfig option, MMC_SKIP_DEINIT, to skip this downgrade for
eMMC devices. SD cards are always deinitialized: a UHS card switched
to 1.8V signaling cannot be reset by CMD0 and needs a power cycle,
which not every board can perform.
CMD0 does reset a card that is at 1.8V - it returns to Idle and keeps
working with SDR12 timing. A power cycle is only needed to go
back to 3.3V signaling.
CMD6 (SWITCH_FUNC - Check) can tell you whether the card is
already at 1.8V. So the OS can decide how to re-init the card rather
than assuming a power cycle is required.
I would suggest rewording this paragraph. Keeping SD always-deinit
is perfectly fine, just not for this reason.
This removes unnecessary CMD6 switches, retuning and clock changes
from the OS-handoff path, which matters on fast-boot / automotive
platforms.
Signed-off-by: Minwoo Park<[email protected]>
---
drivers/mmc/Kconfig | 23 +++++++++++++++++++++++
drivers/mmc/mmc.c | 3 +++
2 files changed, 26 insertions(+)
base-commit: ece349ade2973e220f524ce59e59711cc919263f
--
2.25.1
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diff --git a/drivers/mmc/Kconfig b/drivers/mmc/Kconfig
index 6c513328de2..8b516bf359f 100644
--- a/drivers/mmc/Kconfig
+++ b/drivers/mmc/Kconfig
@@ -226,6 +226,29 @@ config SPL_MMC_HS200_SUPPORT
The HS200 mode is support by some eMMC. The bus frequency is up to
200MHz. This mode requires tuning the IO.
+config MMC_SKIP_DEINIT
The name MMC_SKIP_DEINIT is broader than the code, which only
skips for eMMC. Would MMC_SKIP_EMMC_DEINIT be a better fit?
Your call.
+ bool "Skip eMMC deinit on OS handoff"
+ depends on MMC_HS200_SUPPORT || MMC_HS400_SUPPORT
Is there a reason MMC_HS400_ES_SUPPORT is left out here? If
there's no reason against it, I'd suggest to add it.
While reviewing this I also noticed the outer guard at the top
of mmc_deinit() has the same gap. I'll send a follow-up patch for that.
+ help
+ When U-Boot hands off to the OS, the MMC subsystem normally
+ renegotiates the bus down out of HS200/HS400 by issuing CMD6
+ switches and retuning, then changes the clock. For eMMC this is
+ unnecessary: the OS issues CMD0 (card reset), which returns the
+ device to backward-compatible mode, and renegotiates from scratch
+ regardless of the mode U-Boot left the device in.
+
+ Enabling this option skips the deinit for eMMC devices only. SD
+ cards are always deinitialized: a UHS card switched to 1.8V
+ signaling cannot be reset by CMD0 and needs a power cycle, which
+ not every board can perform.
+
+ This removes unnecessary CMD6 switches, retuning and clock
+ changes from the OS-handoff path, which matters on fast-boot /
+ automotive platforms.
Same as above.
+
+ If unsure, say N. If your eMMC device or controller cannot be
+ re-initialised by the OS out of HS200/HS400, say N.
"If unsure, say N" is fine. The current wording is a bit vague.
Can you rephrase it to something like say Y: only if the OS can
HW-reset the eMMC (RST_n) or power-cycle.
+
config MMC_VERBOSE
bool "Output more information about the MMC"
default y
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index 2e565560656..121739a0a33 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -3184,6 +3184,9 @@ int mmc_deinit(struct mmc *mmc)
return sd_select_mode_and_width(mmc, caps_filtered);
} else {
+ if (CONFIG_IS_ENABLED(MMC_SKIP_DEINIT))
+ return 0;
+
caps_filtered = mmc->card_caps &
~(MMC_CAP(MMC_HS_200) | MMC_CAP(MMC_HS_400) |
MMC_CAP(MMC_HS_400_ES));
