Messages in this thread | | | Subject | Re: dma-api debugfs directory is not created since debugfs is not initialized | From | Robin Murphy <> | Date | Tue, 27 Apr 2021 13:32:50 +0100 |
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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.
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