Commit e9ce2ce1 authored by Ulf Hansson's avatar Ulf Hansson
Browse files

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 <pierre@ossman.eu>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: default avatarUlf Hansson <ulf.hansson@linaro.org>
Reviewed-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20210212131610.236843-1-ulf.hansson@linaro.org
parent fefdd3c9

drivers/mmc/core/core.c

+4 −85
Original line number Diff line number Diff line
@@ -1377,63 +1377,13 @@ 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);

}



/*
@@ -1441,18 +1391,7 @@ void mmc_attach_bus(struct mmc_host *host, const struct mmc_bus_ops *ops) 
 */

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

+0 −2
Original line number Diff line number Diff line
@@ -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;