dma/pool: Improve pool lookup

If CONFIG_ZONE_DMA32 is enabled, but we have not allocated the
corresponding atomic_pool_dma32, dma_guess_pool() may return the NULL
value of that and fail a GFP_DMA32 allocation without trying to fall
back to other pools which may exist. Furthermore, if no GFP_DMA pool
exists, it is preferable to try GFP_DMA32 rather than immediately fall
back to GFP_KERNEL with even less chance of success. Improve matters
by encoding an explicit order of pool preference for each flag.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Vladimir Kondratiev <vladimir.kondratiev@mobileye.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/c846b1a2f43295cac926c7af2ce907f62baec518.1768230104.git.robin.murphy@arm.com

Author: rmurphy-arm

kernel/dma/pool.c

@@ -224,10 +224,10 @@ postcore_initcall(dma_atomic_pool_init);
 static inline struct gen_pool *dma_guess_pool(struct gen_pool *prev, gfp_t gfp)
 {
 	if (prev == NULL) {
-		if (IS_ENABLED(CONFIG_ZONE_DMA32) && (gfp & GFP_DMA32))
-			return atomic_pool_dma32;
-		if (atomic_pool_dma && (gfp & GFP_DMA))
-			return atomic_pool_dma;
+		if (gfp & GFP_DMA)
+			return atomic_pool_dma ?: atomic_pool_dma32 ?: atomic_pool_kernel;
+		if (gfp & GFP_DMA32)
+			return atomic_pool_dma32 ?: atomic_pool_dma ?: atomic_pool_kernel;
 		return atomic_pool_kernel;
 	}
 	if (prev == atomic_pool_kernel)