Re: [PATCH] char: xillybus: use stream_open for stream semantics

[Eli Billauer <eli.billauer@gmail.com>]

Hello Mohammad,

On 31/10/2026 9:36, mdshahid03@gmail.com wrote:
> Use stream_open() to mark these channels as streams and
> disable unnecessary file position handling,

The difference between stream_open() and nonseekable_open() is that the 
former sets FMODE_STREAM and clears FMODE_ATOMIC_POS. I made a quick 
test run, and FMODE_ATOMIC_POS isn't set in either case. As 
FMODE_ATOMIC_POS is clear, no lock is taken while updating fpos either way.

So in effect, the only difference is that FMODE_STREAM is set. Frankly 
speaking, I'm not sure in what way this matters, if at all.

> allowing read
> and write operations to run concurrently.

I checked that with hardware. There's no problem running read() and 
write() concurrently even before this patch. And I don't see why there 
would be a problem.

It seems like FMODE_STREAM was supposed to replace FMODE_ATOMIC_POS 
somewhere back in 2019 with Linus' commit 0be0ee71816b ("vfs: properly 
and reliably lock f_pos in fdget_pos()"). That is where he urged the 
move towards stream_open(). But then he reverted it with commit 
2be7d348fe92 soon afterwards.

All in all, I don't know what to say about this. Calling stream_open() 
does sound more correct, and nothing broke with this patch. But I'm 
generally unhappy fiddling with code unless it fixes something.

Regards,
    Eli