dma-contiguous: hornor the cma address limit setup by user

When porting a cma related usage from x86_64 server to arm64 server,
the "cma=4G@4G" setup failed on arm64. The reason is arm64 and some
other architectures have specific physical address limit for reserved
cma area, like 4GB due to the device's need for 32 bit dma. Actually
lots of platforms of those architectures don't have this device dma
limit, but still have to obey it, and are not able to reserve a huge
cma pool.

This situation could be improved by honoring the user input cma
physical address than the arch limit. As when users specify it, they
already knows what the default is which probably can't suit them.

Suggested-by: Robin Murphy <[email protected]>
Signed-off-by: Feng Tang <[email protected]>
Signed-off-by: Marek Szyprowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Feng Tang

kernel/dma/contiguous.c

@@ -222,7 +222,10 @@ void __init dma_contiguous_reserve(phys_addr_t limit)
 	if (size_cmdline != -1) {
 		selected_size = size_cmdline;
 		selected_base = base_cmdline;
-		selected_limit = min_not_zero(limit_cmdline, limit);
+
+		/* Hornor the user setup dma address limit */
+		selected_limit = limit_cmdline ?: limit;
+
 		if (base_cmdline + size_cmdline == limit_cmdline)
 			fixed = true;
 	} else {