[PATCH v5 4/9] driver core: Replace dev->dma_skip_sync with dev_dma_skip_sync()

Douglas Anderson
2026-04-06T23:25:34Z

threads without locking. Switch "dma_skip_sync" over to the "flags"
field so modifications are safe.

Cc: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Christoph Hellwig <hch@lst.de>
Reviewed-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---
Not fixing any known bugs; problem is theoretical and found by code
inspection. Change is done somewhat manually and only lightly tested
(mostly compile-time tested).

NOTE: even though previously we only took up a bit if
CONFIG_DMA_NEED_SYNC, in this change I reserve the bit
unconditionally. While we could get the "dynamic" behavior by changing
the flags definition to be an unnumbered "enum", Greg has requested
that the numbers be stable.

(no changes since v4)

Changes in v4:
- Use accessor functions for flags

Changes in v3:
- New

include/linux/device.h | 8 ++++----
include/linux/dma-map-ops.h | 4 ++--
include/linux/dma-mapping.h | 2 +-
kernel/dma/mapping.c | 8 ++++----
mm/hmm.c | 2 +-
5 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/include/linux/device.h b/include/linux/device.h
index 620306e8380a..c38593cf2583 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -471,11 +471,14 @@ struct device_physical_location {
* until other devices probe successfully.
* @DEV_FLAG_DMA_IOMMU: Device is using default IOMMU implementation for DMA and
* doesn't rely on dma_ops structure.
+ * @DEV_FLAG_DMA_SKIP_SYNC: DMA sync operations can be skipped for coherent
+ * buffers.
*/
enum struct_device_flags {
DEV_FLAG_READY_TO_PROBE = 0,
DEV_FLAG_CAN_MATCH = 1,
DEV_FLAG_DMA_IOMMU = 2,
+ DEV_FLAG_DMA_SKIP_SYNC = 3,

DEV_FLAG_COUNT
};
@@ -569,7 +572,6 @@ enum struct_device_flags {
* and optionall (if the coherent mask is large enough) also
* for dma allocations. This flag is managed by the dma ops
* instance from ->dma_supported.
- * @dma_skip_sync: DMA sync operations can be skipped for coherent buffers.
* @flags: DEV_FLAG_XXX flags. Use atomic bitfield operations to modify.
*
* At the lowest level, every device in a Linux system is represented by an
@@ -686,9 +688,6 @@ struct device {
#ifdef CONFIG_DMA_OPS_BYPASS
bool dma_ops_bypass : 1;
#endif
-#ifdef CONFIG_DMA_NEED_SYNC
- bool dma_skip_sync:1;
-#endif

DECLARE_BITMAP(flags, DEV_FLAG_COUNT);
};
@@ -718,6 +717,7 @@ static inline bool dev_test_and_set_##accessor_name(struct device *dev) \
__create_dev_flag_accessors(ready_to_probe, DEV_FLAG_READY_TO_PROBE);
__create_dev_flag_accessors(can_match, DEV_FLAG_CAN_MATCH);
__create_dev_flag_accessors(dma_iommu, DEV_FLAG_DMA_IOMMU);
+__create_dev_flag_accessors(dma_skip_sync, DEV_FLAG_DMA_SKIP_SYNC);

#undef __create_dev_flag_accessors

diff --git a/include/linux/dma-map-ops.h b/include/linux/dma-map-ops.h
index 60b63756df82..edd7de60a957 100644
--- a/include/linux/dma-map-ops.h
+++ b/include/linux/dma-map-ops.h
@@ -245,8 +245,8 @@ static inline void dma_reset_need_sync(struct device *dev)
{
#ifdef CONFIG_DMA_NEED_SYNC
/* Reset it only once so that the function can be called on hotpath */
- if (unlikely(dev->dma_skip_sync))
- dev->dma_skip_sync = false;
+ if (unlikely(dev_dma_skip_sync(dev)))
+ dev_clear_dma_skip_sync(dev);
#endif
}

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index 99ef042ecdb4..ef4feb2b37d8 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -419,7 +419,7 @@ bool __dma_need_sync(struct device *dev, dma_addr_t dma_addr);
static inline bool dma_dev_need_sync(const struct device *dev)
{
/* Always call DMA sync operations when debugging is enabled */
- return !dev->dma_skip_sync || IS_ENABLED(CONFIG_DMA_API_DEBUG);
+ return !dev_dma_skip_sync(dev) || IS_ENABLED(CONFIG_DMA_API_DEBUG);
}

static inline void dma_sync_single_for_cpu(struct device *dev, dma_addr_t addr,
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 6d3dd0bd3a88..76fc65d1a8a4 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -467,7 +467,7 @@ bool dma_need_unmap(struct device *dev)
{
if (!dma_map_direct(dev, get_dma_ops(dev)))
return true;
- if (!dev->dma_skip_sync)
+ if (!dev_dma_skip_sync(dev))
return true;
return IS_ENABLED(CONFIG_DMA_API_DEBUG);
}
@@ -483,16 +483,16 @@ static void dma_setup_need_sync(struct device *dev)
* mapping, if any. During the device initialization, it's
* enough to check only for the DMA coherence.
*/
- dev->dma_skip_sync = dev_is_dma_coherent(dev);
+ dev_assign_dma_skip_sync(dev, dev_is_dma_coherent(dev));
else if (!ops->sync_single_for_device && !ops->sync_single_for_cpu &&
!ops->sync_sg_for_device && !ops->sync_sg_for_cpu)
/*
* Synchronization is not possible when none of DMA sync ops
* is set.
*/
- dev->dma_skip_sync = true;
+ dev_set_dma_skip_sync(dev);
else
- dev->dma_skip_sync = false;
+ dev_clear_dma_skip_sync(dev);
}
#else /* !CONFIG_DMA_NEED_SYNC */
static inline void dma_setup_need_sync(struct device *dev) { }
diff --git a/mm/hmm.c b/mm/hmm.c
index 5955f2f0c83d..c72c9ddfdb2f 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -709,7 +709,7 @@ int hmm_dma_map_alloc(struct device *dev, struct hmm_dma_map *map,
* best approximation to ensure no swiotlb buffering happens.
*/
#ifdef CONFIG_DMA_NEED_SYNC
- dma_need_sync = !dev->dma_skip_sync;
+ dma_need_sync = !dev_dma_skip_sync(dev);
#endif /* CONFIG_DMA_NEED_SYNC */
if (dma_need_sync || dma_addressing_limited(dev))
return -EOPNOTSUPP;
--
2.53.0.1213.gd9a14994de-goog