mmc: core: Add infrastructure for undervoltage handling

Implement the core infrastructure to allow MMC bus types to handle
REGULATOR_EVENT_UNDER_VOLTAGE events from power regulators. This is
primarily aimed at allowing devices like eMMC to perform an emergency
shutdown to prevent data corruption when a power failure is imminent.

This patch introduces:
- A new 'handle_undervoltage' function pointer to 'struct mmc_bus_ops'.
  Bus drivers (e.g., for eMMC) can implement this to define their
  emergency procedures.
- A workqueue ('uv_work') in 'struct mmc_supply' to handle the event
  asynchronously in a high-priority context.
- A new function 'mmc_handle_undervoltage()' which is called from the
  workqueue. It stops the host queue to prevent races with card removal,
  checks for the bus op, and invokes the handler.
- Functions to register and unregister the regulator notifier, intended
  to be called by bus drivers like 'mmc_attach_mmc' when a compatible
  card is detected.

The notifier is only registered for the main vmmc supply, as
undervoltage handling for vqmmc or vqmmc2 is not required at this
time.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20250821130751.2089587-2-o.rempel@pengutronix.de
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
index 88fd231fee1d..860378bea557 100644
--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -1398,6 +1398,29 @@ void mmc_power_cycle(struct mmc_host *host, u32 ocr)
 	mmc_power_up(host, ocr);
 }
 
+/**
+ * mmc_handle_undervoltage - Handle an undervoltage event on the MMC bus
+ * @host: The MMC host that detected the undervoltage condition
+ *
+ * This function is called when an undervoltage event is detected on one of
+ * the MMC regulators.
+ *
+ * Returns: 0 on success or a negative error code on failure.
+ */
+int mmc_handle_undervoltage(struct mmc_host *host)
+{
+	/* Stop the host to prevent races with card removal */
+	__mmc_stop_host(host);
+
+	if (!host->bus_ops || !host->bus_ops->handle_undervoltage)
+		return 0;
+
+	dev_warn(mmc_dev(host), "%s: Undervoltage detected, initiating emergency stop\n",
+		 mmc_hostname(host));
+
+	return host->bus_ops->handle_undervoltage(host);
+}
+
 /*
  * Assign a mmc bus handler to a host. Only one bus handler may control a
  * host at any given time.