dma-coherent: Warn if OF reserved memory is beyond current coherent DMA mask

commit 89461db
Author: Chen-Yu Tsai <[email protected]>
Date: Mon Apr 21 16:39:29 2025 +0800

    dma-coherent: Warn if OF reserved memory is beyond current coherent DMA mask

    When a reserved memory region described in the device tree is attached
    to a device, it is expected that the device's limitations are correctly
    included in that description.

    However, if the device driver failed to implement DMA address masking
    or addressing beyond the default 32 bits (on arm64), then bad things
    could happen because the DMA address was truncated, such as playing
    back audio with no actual audio coming out, or DMA overwriting random
    blocks of kernel memory.

    Check against the coherent DMA mask when the memory regions are attached
    to the device. Give a warning when the memory region can not be covered
    by the mask.

    A warning instead of a hard error was chosen, because it is possible
    that existing drivers could be working fine even if they forgot to
    extend the coherent DMA mask.

    Signed-off-by: Chen-Yu Tsai <[email protected]>
    Signed-off-by: Marek Szyprowski <[email protected]>
    Link: https://lore.kernel.org/r/[email protected]

(cherry picked from commit 89461db)
Signed-off-by: Jerry Snitselaar <[email protected]>
Upstream-Status: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
JIRA: https://issues.redhat.com/browse/RHEL-89891

Author: snits

kernel/dma/coherent.c

@@ -335,16 +335,22 @@ static struct reserved_mem *dma_reserved_default_memory __initdata;
 
 static int rmem_dma_device_init(struct reserved_mem *rmem, struct device *dev)
 {
-	if (!rmem->priv) {
-		struct dma_coherent_mem *mem;
+	struct dma_coherent_mem *mem = rmem->priv;
 
+	if (!mem) {
 		mem = dma_init_coherent_memory(rmem->base, rmem->base,
 					       rmem->size, true);
 		if (IS_ERR(mem))
 			return PTR_ERR(mem);
 		rmem->priv = mem;
 	}
-	dma_assign_coherent_memory(dev, rmem->priv);
+
+	/* Warn if the device potentially can't use the reserved memory */
+	if (mem->device_base + rmem->size - 1 >
+	    min_not_zero(dev->coherent_dma_mask, dev->bus_dma_limit))
+		dev_warn(dev, "reserved memory is beyond device's set DMA address range\n");
+
+	dma_assign_coherent_memory(dev, mem);
 	return 0;
 }