x86: add DMA support

Both interconnect and PCI are cache coherent on x86, so we shouldn't
need any special CPU barriers for DMA. Indeed, Linux defined neither
ARCH_HAS_SYNC_DMA_FOR_CPU nor ARCH_HAS_SYNC_DMA_FOR_DEVICE on x86.

It thus seems that the only reordering we need to take care of is
compiler-induced reordering. The Linux memory model that barebox adheres
to as well demands that all accesses to shared data are volatile.

volatile accesses are already guarnateed to not be reordered against
each other, so we don't even need an explicit barrier(), which is
already the case on other architectures that have a disabled MMU.

Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Link: https://lore.barebox.org/20210416062436.332665-2-ahmad@a3f.at
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

2 files changed, 36 insertions, 1 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 311c3d1a8e..bcb44b23f0 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -4,6 +4,7 @@
 config X86
 	bool
 	select HAS_KALLSYMS
+	select HAS_DMA
 	select GENERIC_FIND_NEXT_BIT
 	default y
 
diff --git a/arch/x86/include/asm/dma.h b/arch/x86/include/asm/dma.h
index 3dab2b688d..8a3b044f3a 100644
--- a/arch/x86/include/asm/dma.h
+++ b/arch/x86/include/asm/dma.h
@@ -4,6 +4,40 @@
 #ifndef __ASM_DMA_H
 #define __ASM_DMA_H
 
-/* empty */
+#include <linux/string.h>
+#include <linux/compiler.h>
+#include <xfuncs.h>
+#include <malloc.h>
+
+/*
+ * x86 is cache coherent, so we need not do anything special here
+ */
+
+static inline void *dma_alloc_coherent(size_t size, dma_addr_t *dma_handle)
+{
+	void *ret = xmemalign(4096, size);
+	if (dma_handle)
+		*dma_handle = (dma_addr_t)ret;
+
+	memset(ret, 0, size);
+
+	return ret;
+}
+
+static inline void dma_free_coherent(void *mem, dma_addr_t dma_handle,
+				     size_t size)
+{
+	free(mem);
+}
+
+static inline void dma_sync_single_for_cpu(dma_addr_t address, size_t size,
+					   enum dma_data_direction dir)
+{
+}
+
+static inline void dma_sync_single_for_device(dma_addr_t address, size_t size,
+					      enum dma_data_direction dir)
+{
+}
 
 #endif /* __ASM_DMA_H */