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, 11 insertions, 0 deletions

diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index 5ed5d203de23..e0d935a4ac1d 100644
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -337,11 +337,15 @@ struct mmc_slot {
 
 struct regulator;
 struct mmc_pwrseq;
+struct notifier_block;
 
 struct mmc_supply {
 	struct regulator *vmmc;		/* Card power supply */
 	struct regulator *vqmmc;	/* Optional Vccq supply */
 	struct regulator *vqmmc2;	/* Optional supply for phy */
+
+	struct notifier_block vmmc_nb;	/* Notifier for vmmc */
+	struct work_struct uv_work;	/* Undervoltage work */
 };
 
 struct mmc_ctx {
@@ -494,6 +498,13 @@ struct mmc_host {
 	unsigned int		can_dma_map_merge:1; /* merging can be used */
 	unsigned int		vqmmc_enabled:1; /* vqmmc regulator is enabled */
 
+	/*
+	 * Indicates if an undervoltage event has already been handled.
+	 * This prevents repeated regulator notifiers from triggering
+	 * multiple REGULATOR_EVENT_UNDER_VOLTAGE events.
+	 */
+	unsigned int		undervoltage:1;	 /* Undervoltage state */
+
 	int			rescan_disable;	/* disable card detection */
 	int			rescan_entered;	/* used with nonremovable devices */