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

Robin Murphy
2021-04-27T12:33:03Z

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.