mmc: core: Drop reference counting of the bus_ops

When the mmc_rescan work is enabled for execution (host->rescan_disable),
it's the only instance per mmc host that is allowed to set/clear the
host->bus_ops pointer.

Besides the mmc_rescan work, there are a couple of scenarios when the
host->bus_ops pointer may be accessed. Typically, those can be described as
as below:

*)
Upper mmc driver layers (like the mmc block device driver or an SDIO
functional driver) needs to execute a host->bus_ops callback. This can be
considered as safe without having to use some special locking mechanism,
because they operate on top of the struct mmc_card. As long as there is a
card to operate upon, the mmc core guarantees that there is a host->bus_ops
assigned as well. Note that, upper layer mmc drivers are of course
responsible to clean up from themselves from their ->remove() callbacks,
otherwise things would fall apart anyways.

**)
Via the mmc host instance, we may need to force a removal of an inserted
mmc card. This happens when a mmc host driver gets unbind, for example. In
this case, we protect the host->bus_ops pointer from concurrent accesses,
by disabling the mmc_rescan work upfront (host->rescan_disable). See
mmc_stop_host() for example.

This said, it seems like the reference counting of the host->bus_ops
pointer at some point have become superfluous. As this is an old mechanism
of the mmc core, it a bit difficult to digest the history of when that
could have happened. However, let's drop the reference counting to avoid
unnecessary code-paths and lockings.

Cc: Pierre Ossman <[email protected]>
Cc: Russell King <[email protected]>
Signed-off-by: Ulf Hansson <[email protected]>
Reviewed-by: Adrian Hunter <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

drivers/mmc/core/core.c

@@ -1377,82 +1377,21 @@ void mmc_power_cycle(struct mmc_host *host, u32 ocr)
 	mmc_power_up(host, ocr);
 }
 
-/*
- * Cleanup when the last reference to the bus operator is dropped.
- */
-static void __mmc_release_bus(struct mmc_host *host)
-{
-	WARN_ON(!host->bus_dead);
-
-	host->bus_ops = NULL;
-}
-
-/*
- * Increase reference count of bus operator
- */
-static inline void mmc_bus_get(struct mmc_host *host)
-{
-	unsigned long flags;
-
-	spin_lock_irqsave(&host->lock, flags);
-	host->bus_refs++;
-	spin_unlock_irqrestore(&host->lock, flags);
-}
-
-/*
- * Decrease reference count of bus operator and free it if
- * it is the last reference.
- */
-static inline void mmc_bus_put(struct mmc_host *host)
-{
-	unsigned long flags;
-
-	spin_lock_irqsave(&host->lock, flags);
-	host->bus_refs--;
-	if ((host->bus_refs == 0) && host->bus_ops)
-		__mmc_release_bus(host);
-	spin_unlock_irqrestore(&host->lock, flags);
-}
-
 /*
  * Assign a mmc bus handler to a host. Only one bus handler may control a
  * host at any given time.
  */
 void mmc_attach_bus(struct mmc_host *host, const struct mmc_bus_ops *ops)
 {
-	unsigned long flags;
-
-	WARN_ON(!host->claimed);
-
-	spin_lock_irqsave(&host->lock, flags);
-
-	WARN_ON(host->bus_ops);
-	WARN_ON(host->bus_refs);
-
 	host->bus_ops = ops;
-	host->bus_refs = 1;
-	host->bus_dead = 0;
-
-	spin_unlock_irqrestore(&host->lock, flags);
 }
 
 /*
  * Remove the current bus handler from a host.
  */
 void mmc_detach_bus(struct mmc_host *host)
 {
-	unsigned long flags;
-
-	WARN_ON(!host->claimed);
-	WARN_ON(!host->bus_ops);
-
-	spin_lock_irqsave(&host->lock, flags);
-
-	host->bus_dead = 1;
-
-	spin_unlock_irqrestore(&host->lock, flags);
-
-	mmc_bus_put(host);
+	host->bus_ops = NULL;
 }
 
 void _mmc_detect_change(struct mmc_host *host, unsigned long delay, bool cd_irq)
@@ -2244,32 +2183,15 @@ void mmc_rescan(struct work_struct *work)
 		host->trigger_card_event = false;
 	}
 
-	mmc_bus_get(host);
-
 	/* Verify a registered card to be functional, else remove it. */
-	if (host->bus_ops && !host->bus_dead)
+	if (host->bus_ops)
 		host->bus_ops->detect(host);
 
 	host->detect_change = 0;
 
-	/*
-	 * Let mmc_bus_put() free the bus/bus_ops if we've found that
-	 * the card is no longer present.
-	 */
-	mmc_bus_put(host);
-	mmc_bus_get(host);
-
 	/* if there still is a card present, stop here */
-	if (host->bus_ops != NULL) {
-		mmc_bus_put(host);
+	if (host->bus_ops != NULL)
 		goto out;
-	}
-
-	/*
-	 * Only we can add a new handler, so it's safe to
-	 * release the lock here.
-	 */
-	mmc_bus_put(host);
 
 	mmc_claim_host(host);
 	if (mmc_card_is_removable(host) && host->ops->get_cd &&
@@ -2332,18 +2254,15 @@ void mmc_stop_host(struct mmc_host *host)
 	/* clear pm flags now and let card drivers set them as needed */
 	host->pm_flags = 0;
 
-	mmc_bus_get(host);
-	if (host->bus_ops && !host->bus_dead) {
+	if (host->bus_ops) {
 		/* Calling bus_ops->remove() with a claimed host can deadlock */
 		host->bus_ops->remove(host);
 		mmc_claim_host(host);
 		mmc_detach_bus(host);
 		mmc_power_off(host);
 		mmc_release_host(host);
-		mmc_bus_put(host);
 		return;
 	}
-	mmc_bus_put(host);
 
 	mmc_claim_host(host);
 	mmc_power_off(host);

include/linux/mmc/host.h

@@ -423,7 +423,6 @@ struct mmc_host {
 	/* group bitfields together to minimize padding */
 	unsigned int		use_spi_crc:1;
 	unsigned int		claimed:1;	/* host exclusively claimed */
-	unsigned int		bus_dead:1;	/* bus has been released */
 	unsigned int		doing_init_tune:1; /* initial tuning in progress */
 	unsigned int		can_retune:1;	/* re-tuning can be used */
 	unsigned int		doing_retune:1;	/* re-tuning in progress */
@@ -454,7 +453,6 @@ struct mmc_host {
 	struct mmc_slot		slot;
 
 	const struct mmc_bus_ops *bus_ops;	/* current bus driver */
-	unsigned int		bus_refs;	/* reference counter */
 
 	unsigned int		sdio_irqs;
 	struct task_struct	*sdio_irq_thread;