linux-kernel - Re: inverse mapping from a struct console to device

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Message-ID: <20150126202031.GA14511@kroah.com>
Date:	Mon, 26 Jan 2015 12:20:31 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Jon Masters <jcm@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: inverse mapping from a struct console to device

On Mon, Jan 26, 2015 at 02:40:58PM -0500, Jon Masters wrote:
> Hi Folks,
> 
> TLDR: I need a back reference from a console struct to its device.

I know I'm going to regret answering this vague question, but, what
_exactly_ do you mean by "its device"?

> I can't see an easy way to do this right now without adding one?

>From within the kernel or from userspace?

If in the kernel, what part of the kernel are you doing this from?

If in userspace, what is wrong with /sys/class/tty/console?

> I've a quick question. I have prototype code that parses an ACPI table
> known as the SPCR (Serial Port Console Redirection - exists on both x86
> and ARM systems). It finds the correct serial device (but it's not a
> Linux specific DT-style solution so there's no "console=" parameter
> embedded in it or something) to use for the preferred console (I have
> effectively the device object connected with the acpi_handle of the
> underlying serial device, which I map on by walking the ACPI namespace
> down from the root until I find the _ADR of the specified SPCR address).
> 
> One of the problems that I have is that I want to make the decision
> about whether a console should be automatically configured once we get
> to register_console, because that's where the existing logic is, and
> it's one path I can be sure everyone is going to call (serial drivers
> have a number of different APIs that they use during registration).
> 
> Once I'm in register_console, I have only the console struct to play
> with. I can get from that to the tty_driver (by walking the tty driver
> list and finding the back reference), but that still doesn't give me
> what I want. I can add a reference to the underlying device in one of
> these structs, but I would like to have additional opinions before I go
> and make something hackish folks will balk at later. So, am I missing
> something obvious? I hope I am because this seems slightly weird.

I really don't understand what you are wanting to do here.  Who wants to
"automatically configure" a console?

> P.S. I would love it if I could (for debug purposes only) just walk down
> from the root of the kernel device hierarchy until I found a device that
> had a resource matching the one I am looking for (you can do exactly
> this in the ACPI namespace using exported methods), but the only code
> that can do that today is unexported core core used during shutdown to
> shutdown all devices recursively. Just a sidenote.

Sorry, you don't know what "type" of devices you are walking through, so
bad things can happen if you guess wrong.

greg k-h
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