linux-kernel - RE: [EXT] Re: [PATCH] MA-21654 Use dma_alloc_pages in vb2_dma_sg_alloc

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Date:   Wed, 20 Sep 2023 10:02:35 +0000
From:   Hui Fang <hui.fang@....com>
To:     Tomasz Figa <tfiga@...omium.org>, Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>
CC:     "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "mchehab@...nel.org" <mchehab@...nel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Anle Pan <anle.pan@....com>, Xuegang Liu <xuegang.liu@....com>
Subject: RE: [EXT] Re: [PATCH] MA-21654 Use dma_alloc_pages in
 vb2_dma_sg_alloc_compacted

On Thu, Sep 20, 2023 at 15:41 PM Tomasz Figa <tfiga@...omium.org> wrote:

> Is CONFIG_ZONE_DMA32 really the factor that triggers the problem? My
> understanding was that the problem was that the hardware has 32-bit DMA,
> but the system has physical memory at addresses beyond the first 4G.

Yes, you are right. But CONFIG_ZONE_DMA32 may affect swiotlb_init_remap().

In arch/arm64/mm/init.c
static void __init zone_sizes_init(void)
{
......
#ifdef CONFIG_ZONE_DMA32
	max_zone_pfns[ZONE_DMA32] = disable_dma32 ? 0 : PFN_DOWN(dma32_phys_limit);
	if (!arm64_dma_phys_limit)
		arm64_dma_phys_limit = dma32_phys_limit;
#endif
......
}


void __init mem_init(void)
{
	swiotlb_init(max_pfn > PFN_DOWN(arm64_dma_phys_limit), SWIOTLB_VERBOSE);
}

In kernel/dma/swiotlb.c

void __init swiotlb_init(bool addressing_limit, unsigned int flags)
{
	swiotlb_init_remap(addressing_limit, flags, NULL);
}

void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
		int (*remap)(void *tlb, unsigned long nslabs))
{
	struct io_tlb_mem *mem = &io_tlb_default_mem;
	unsigned long nslabs;
	size_t alloc_size;
	size_t bytes;
	void *tlb;

	if (!addressing_limit && !swiotlb_force_bounce)
  		return;
}

Also thanks for your suggestion, will refine my patch.

BRs,
Fang Hui