[bug report] dma-buf: Add debug option

[yf.wang]

Hi Daniel Vetter,

The patch https://patchwork.freedesktop.org/patch/414455/:
"dma-buf: Add debug option" from Jan. 15, 2021, leads to the following expection:

Backtrace:

[<ffffffc0081a2258>] atomic_notifier_call_chain+0x9c/0xe8
[<ffffffc0081a2d54>] notify_die+0x114/0x19c
[<ffffffc0080348d8>] __die+0xec/0x468
[<ffffffc008034648>] die+0x54/0x1f8
[<ffffffc0080631e8>] die_kernel_fault+0x80/0xbc
[<ffffffc0080630fc>] __do_kernel_fault+0x268/0x2d4
[<ffffffc008062c4c>] do_bad_area+0x68/0x148
[<ffffffc00a6dab34>] do_translation_fault+0xbc/0x108
[<ffffffc0080619f8>] do_mem_abort+0x6c/0x1e8
[<ffffffc00a68f5cc>] el1_abort+0x3c/0x64
[<ffffffc00a68f54c>] el1h_64_sync_handler+0x5c/0xa0
[<ffffffc008011ae4>] el1h_64_sync+0x78/0x80
[<ffffffc008063b9c>] dcache_inval_poc+0x40/0x58
[<ffffffc009236104>] iommu_dma_sync_sg_for_cpu+0x144/0x280
[<ffffffc0082b4870>] dma_sync_sg_for_cpu+0xbc/0x110
[<ffffffc002c7538c>] system_heap_dma_buf_begin_cpu_access+0x144/0x1e0 [system_heap]
[<ffffffc0094154e4>] dma_buf_begin_cpu_access+0xa4/0x10c
[<ffffffc004888df4>] isp71_allocate_working_buffer+0x3b0/0xe8c [mtk_hcp]
[<ffffffc004884a20>] mtk_hcp_allocate_working_buffer+0xc0/0x108 [mtk_hcp]

Because of CONFIG_DMABUF_DEBUG will default enable when DMA_API_DEBUG enable,
and when not support dma coherent, since the main function of user calling
dma_buf_begin_cpu_access and dma_buf_end_cpu_access is to do cache sync during
dma_buf_map_attachment and dma_buf_unmap_attachment, which get PA error from
sgtable by sg_phys(sg), this leads to the expection.

1.dma_buf_map_attachement()
 -.> mangle_sg_table(sg)  // "sg->page_link ^= ~0xffUL" to rotate PA in this patch.

2.dma_buf_begin_cpu_access()
 -.> system_heap_dma_buf_begin_cpu_access() in system_heap.c  // do cache sync if mapped attachment before
    -.> iommu_dma_sync_sg_for_cpu() in dma-iommu.c
        -.>  arch_sync_dma_for_device(sg_phys(sg), sg->length, dir) // get PA error since PA mix up

3.dma_buf_end_cpu_access() and dma_buf_begin_cpu_access are similar.

4.dma_buf_unmap_attachement()
       -.> mangle_sg_table(sg) // "sg->page_link ^= ~0xffUL" to rotate PA



drivers/dma-buf/Kconfig:
config DMABUF_DEBUG
	bool "DMA-BUF debug checks"
	default y if DMA_API_DEBUG


drivers/dma-buf/dma-buf.c:
static void mangle_sg_table(struct sg_table *sg_table)
{
#ifdef CONFIG_DMABUF_DEBUG
	int i;
	struct scatterlist *sg;

	/* To catch abuse of the underlying struct page by importers mix
	 * up the bits, but take care to preserve the low SG_ bits to
	 * not corrupt the sgt. The mixing is undone in __unmap_dma_buf
	 * before passing the sgt back to the exporter. */
	for_each_sgtable_sg(sg_table, sg, i)
		sg->page_link ^= ~0xffUL;
#endif
}


drivers/iommu/dma-iommu.c:
static void iommu_dma_sync_sg_for_cpu(struct device *dev,
		struct scatterlist *sgl, int nelems,
		enum dma_data_direction dir)
{
	struct scatterlist *sg;
	int i;

	if (dev_is_dma_coherent(dev) && !dev_is_untrusted(dev))
		return;

	for_each_sg(sgl, sg, nelems, i) {
		if (!dev_is_dma_coherent(dev))
			arch_sync_dma_for_cpu(sg_phys(sg), sg->length, dir);

		if (is_swiotlb_buffer(sg_phys(sg)))
			swiotlb_tbl_sync_single(dev, sg_phys(sg), sg->length,
						dir, SYNC_FOR_CPU);
	}
}


Thanks,
Yunfei.