[PATCH 5.10 475/491] mmc: core: Drop reference counting of the bus_ops

Greg Kroah-Hartman
2026-04-13T17:03:10Z

From: Ulf Hansson <ulf.hansson@linaro.org>

[ Upstream commit e9ce2ce17da626d930812199568bd426b2832f57 ]

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: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20210212131610.236843-1-ulf.hansson@linaro.org
Stable-dep-of: 901084c51a0a ("mmc: core: Avoid bitfield RMW for claim/retune flags")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/mmc/core/core.c | 89 ++---------------------------------------------
include/linux/mmc/host.h | 2 -
2 files changed, 4 insertions(+), 87 deletions(-)

--- a/drivers/mmc/core/core.c
+++ b/drivers/mmc/core/core.c
@@ -1389,62 +1389,12 @@ void mmc_power_cycle(struct mmc_host *ho
}

/*
- * 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);
}

/*
@@ -1452,18 +1402,7 @@ void mmc_attach_bus(struct mmc_host *hos
*/
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)
@@ -2260,32 +2199,15 @@ 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 &&
@@ -2356,18 +2278,15 @@ 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);
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -403,7 +403,6 @@ 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 */
@@ -435,7 +434,6 @@ 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;