linux-kernel - Re: [PATCH 1/2] User access to internal clocks

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Message-ID: <20090209175514.GB19453@n2100.arm.linux.org.uk>
Date:	Mon, 9 Feb 2009 17:55:14 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Davide Rizzo <elpa.rizzo@...il.com>
Cc:	gregkh@...e.de, linux-kernel@...r.kernel.org, ben-linux@...ff.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH 1/2] User access to internal clocks

On Mon, Feb 09, 2009 at 06:44:40PM +0100, Davide Rizzo wrote:
> >> This driver is for user level programs to interact with system clocks.
> >> It allows to read and modify rates and parents, using virtual files.
> >> It requires the implementation of 2 additional functions in the clk interface:
> >>  clk_for_each() and clk_name().
> >> Actually I implemented that functions only for Samsung S3C24xx platform.
> >
> > NAK.
> >
> >> +       name = clk_get_name(clk);
> >
> > This implies that there is a 1:1 relationship between a 'name' and a
> > struct clk.  No such thing exists (and where it does, it's being
> > eliminated in ARM because it's just plain and simple WRONG.)
>
> Not exactly, clk_get() could work in both way: a struct clk can be
> exactly identified by name and device id (like clk_get is acting now)
> or by full name, in the format clkname.device

You're not understanding the issue(s).

1. there is no 1:1 mapping between the identifiers (struct device +
   connection ID) and the struct clk.  It's actually a many-to-one
   mapping.

   That means there is _no_ name associated with a struct clk.

   Conceptually, clk_get() gives you a struct clk for the struct device
   and a connection ID.  How that mapping is achieved isn't specified
   in the API, all that's required is that such a mapping is performed.
   It is _specifically_ intended that more than one set of {device,id}
   pairs will map to the same clk.

   So, now to insist that you can go from a struct clk to some kind of
   string identifier for it is changing this - you're now requiring
   that every struct clk has a unique name.  This is not the case.
   PXA, for instance, struct clk's are now completely nameless.  They
   have no identifier.

   If you want to have a string identifier which works in every case,
   this will:

	sprintf(identifer, "%p", clk);

   Or, I guess you could force every struct clk to have a 'sysfs_name'
   field just to export them out via sysfs - and that'll be all that
   it's used for.

2. there is no generic way to walk a set of struct clk's - indeed, there
   may be no list of them (and there exists implmentations where that is
   true) and the only list which does exist is a set ID to clk mapping
   structures.

> Otherwise, what do you suggest to enumerate and distinguish all system
> clocks ?

There exists no such concept in the API, what you're asking for is
implementation specific.
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