[PATCH] New config added to handle 64-bit systems with 32-bit DMA support

[Alberto Secondi]

From: Alberto Secondi <alberto.secondi@xxxxxxxxxxxx>

The kernel assumes that 64-bit systems have 64-bit DMA support through
CONFIG_ARCH_DMA_ADDR_T_64BIT. This is not always true; for example, several
iMX8 systems (verified on iMX8MM and iMX8MP) have DMA with only 32-bit support.
This results in several drivers requesting DMA_BIT_MASK(64), which causes
malfunctions, particularly when systems have more than 3GB of DRAM (verified
with the lan743x driver and iMX8 systems with 4GB of DRAM). Therefore, a new
config ARCH_64BIT_HAS_DMA32_ONLY was added to manage 64-bit systems with 32-bit
DMA, which adjusts DMA_BIT_MASK(n) accordingly.

Signed-off-by: Alberto Secondi <alberto.secondi@xxxxxxxxxxxx>
Co-developed-by: Davide Salaris <davide.salaris@xxxxxxxxxxxx>
---
 include/linux/dma-mapping.h | 4 ++++
 kernel/dma/Kconfig          | 8 ++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index f693aafe221f..629220a777e3 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -74,7 +74,11 @@
  */
 #define DMA_MAPPING_ERROR		(~(dma_addr_t)0)
 
+#ifdef CONFIG_ARCH_64BIT_HAS_DMA32_ONLY
+#define DMA_BIT_MASK(n)	(((n) > 32) ? ((1ULL<<(32))-1) : ((1ULL<<(n))-1))
+#else
 #define DMA_BIT_MASK(n)	(((n) == 64) ? ~0ULL : ((1ULL<<(n))-1))
+#endif
 
 #ifdef CONFIG_DMA_API_DEBUG
 void debug_dma_mapping_error(struct device *dev, dma_addr_t dma_addr);
diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig
index c06e56be0ca1..0a27eafed808 100644
--- a/kernel/dma/Kconfig
+++ b/kernel/dma/Kconfig
@@ -36,6 +36,14 @@ config NEED_DMA_MAP_STATE
 config ARCH_DMA_ADDR_T_64BIT
 	def_bool 64BIT || PHYS_ADDR_T_64BIT
 
+config ARCH_64BIT_HAS_DMA32_ONLY
+        bool "64bit System has DMA32 only"
+        depends on ARCH_DMA_ADDR_T_64BIT
+        default n
+	help
+	  This enables forcing the maximum DMA_BIT_MASK to 32 bits for
+	  64-bit systems that have DMA support limited to 32 bits.
+
 config ARCH_HAS_DMA_SET_MASK
 	bool
 
-- 
2.34.1