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From:   Alexey Skidanov <alexey.skidanov@gmail.com>
Date:   Sun, 28 Jan 2018 18:24:20 +0200
Message-ID: <CAM7Y4U8FmTNrn=BYN-J+_hVz7JTuaiV+y5Bao78wj3vYCPrz+w@mail.gmail.com>
Subject: staging: ion: ION allocation fall back order depends on heap linkage order
To:     devel@driverdev.osuosl.org
Cc:     linux-kernel@vger.kernel.org
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Sender: linux-kernel-owner@vger.kernel.org
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Hi,

According to my understanding, the allocation fall back order
completely depends on heap->id that is assigned during the heap
creation:
   plist_for_each_entry(heap, &dev->heaps, node) {
        /* if the caller didn't specify this heap id */
        if (!((1 << heap->id) & heap_id_mask))
            continue;
        buffer = ion_buffer_create(heap, dev, len, flags);
        if (!IS_ERR(buffer))
            break;
    }

On creation, each heap is added to the priority list according to the
priority assigned:

...
static int heap_id;
...
void ion_device_add_heap(struct ion_heap *heap)
{
        ...
        heap->id = heap_id++;
        ...
}


The order of creation is the order of linkage defined in the Makefile.
Thus, by default, we have:

heap id 2, type ION_HEAP_TYPE_DMA
heap id 1, type ION_HEAP_TYPE_SYSTEM
heap id 0, type ION_HEAP_TYPE_SYSTEM_CONTIG

Changing the linkage order:
diff --git a/drivers/staging/android/ion/Makefile
b/drivers/staging/android/ion/Makefile
index bb30bf8..e05052c 100644
--- a/drivers/staging/android/ion/Makefile
+++ b/drivers/staging/android/ion/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_ION) +=   ion.o ion-ioctl.o ion_heap.o
-obj-$(CONFIG_ION_SYSTEM_HEAP) += ion_system_heap.o ion_page_pool.o
 obj-$(CONFIG_ION_CARVEOUT_HEAP) += ion_carveout_heap.o
 obj-$(CONFIG_ION_CHUNK_HEAP) += ion_chunk_heap.o
 obj-$(CONFIG_ION_CMA_HEAP) += ion_cma_heap.o
+obj-$(CONFIG_ION_SYSTEM_HEAP) += ion_system_heap.o ion_page_pool.o

I get the following order:

heap id 2, type ION_HEAP_TYPE_SYSTEM
heap id 1, type ION_HEAP_TYPE_SYSTEM_CONTIG
heap id 0, type ION_HEAP_TYPE_DMA

So, if the user specifies more than 1 heap in the heap_id_mask during
allocation, the allocation fall back order completely depends on the
order of linkage. Probably, it's better to let the user to define the
fall back order (and NOT to be dependent on the linkage order at all)
?

Thanks,
Alexey