linux-kernel - Re: [PATCH] kernel: dma: let dma use vmalloc area

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Date:   Mon, 27 Nov 2023 10:32:10 +0100
From:   Christoph Hellwig <hch@....de>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        "zhaoyang.huang" <zhaoyang.huang@...soc.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, steve.kang@...soc.com
Subject: Re: [PATCH] kernel: dma: let dma use vmalloc area

On Mon, Nov 27, 2023 at 04:56:45PM +0800, Zhaoyang Huang wrote:
> Sorry for the confusion, please find below codes for more information.
> dma_init_coherent_memory
>     memremap
>         addr = ioremap_wt(offset, size);
> What I mean is addr is a vmalloc address, which is implicitly mapped
> by dma's framework and not be aware of to the driver.

Yes.  And it is only returned from dma_alloc_coherent, which should
never be passed to dma_map_<anything>.

> Please correct me if I am wrong. According to my understanding, cache
> consistency could be solved inside dma_map_page via either
> dma_direct_map_page(swio/arch_sync_dma_for_device) or ops->map_page.
> The original thought of rejecting vmalloc is that this pa is not safe
> as this mapping could go in any time. What I am suggesting is to let
> this kind of va be enrolled.

But that only works for the direct mapping.  It does not work for the
additional aliases created by vmap/ioremap/memremap.  Now that only
matters if the cache is virtually indexed, which is rather unusual
these days.