linux-kernel - Re: dma-api debugfs directory is not created since debugfs is not initialized

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Date:   Tue, 27 Apr 2021 13:32:50 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Corentin Labbe <clabbe.montjoie@...il.com>
Cc:     rafael@...nel.org, hch@....de, m.szyprowski@...sung.com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: dma-api debugfs directory is not created since debugfs is not
 initialized

On 2021-04-27 12:39, Greg KH wrote:
> On Tue, Apr 27, 2021 at 01:34:27PM +0200, Corentin Labbe wrote:
>> Hello
>>
>> I try to debug some DMA problem on next-20210427, and so I have enabled CONFIG_DMA_API_DEBUG=y.
>> But the dma-api directory does show up in debugfs, but lot of other directory exists in it.
> 
> Does it show up properly in 5.12?
> 
>> After debugging it seems due to commit 56348560d495 ("debugfs: do not attempt to create a new file before the filesystem is initalized")
>> Reverting the commit permit to "dma-api" debugfs to be found. (but seems not the right way to fix it).
> 
> We have had some odd start-up ordering issues that the above commit has
> caused to show.  Given that this commit is now in stable kernels, with
> no report of this issue so far, I'm worried that maybe this is a dma
> subsystem ordering issue?

Both debugfs_init() and dma_debug_init() do run at core_initcall level, 
and disassembling the vmlinux from my current working tree does indeed 
suggest that they somehow end up in the wrong relative order:

[...]
ffff80001160d0c8 <__initcall__kmod_debug__325_918_dma_debug_init1>:
ffff80001160d0c8:       feb0d528        .word   0xfeb0d528

[...]

ffff80001160d108 <__initcall__kmod_debugfs__357_848_debugfs_init1>:
ffff80001160d108:       fff4326c        .word   0xfff4326c
[...]


I always had the impression that initcall ordering tended to work out 
roughly alphabetical, such that entries from fs/* might come before 
kernel/*, but I guess it's at the whims of the linker in the end :/

Perhaps the easiest thing to do is split out dma_debug_fs_init() and run 
that at a later level? We do want the dma-debug infrastructure itself to 
be up as early as possible, but I think the debugfs view of its 
internals can happily wait until closer to the time that there's 
actually a userspace to be able to look at it.

Robin.