linux-kernel - Re: [PATCH v4 6/9] clk: Allow parents to be specified without string names

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Message-ID: <155571197779.15276.7804294833146212666@swboyd.mtv.corp.google.com>
Date:   Fri, 19 Apr 2019 15:12:57 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Russell King <linux@...linux.org.uk>,
        Jeffrey Hugo <jhugo@...eaurora.org>,
        Chen-Yu Tsai <wens@...e.org>, Rob Herring <robh@...nel.org>
Subject: Re: [PATCH v4 6/9] clk: Allow parents to be specified without string names

Quoting Stephen Boyd (2019-04-12 11:31:47)
> The common clk framework is lacking in ability to describe the clk
> topology without specifying strings for every possible parent-child
> link. There are a few drawbacks to the current approach:
> 
>  1) String comparisons are used for everything, including describing
>  topologies that are 'local' to a single clock controller.
> 
>  2) clk providers (e.g. i2c clk drivers) need to create globally unique
>  clk names to avoid collisions in the clk namespace, leading to awkward
>  name generation code in various clk drivers.
> 
>  3) DT bindings may not fully describe the clk topology and linkages
>  between clk controllers because drivers can easily rely on globally unique
>  strings to describe connections between clks.
> 
> This leads to confusing DT bindings, complicated clk name generation
> code, and inefficient string comparisons during clk registration just so
> that the clk framework can detect the topology of the clk tree.
> Furthermore, some drivers call clk_get() and then __clk_get_name() to
> extract the globally unique clk name just so they can specify the parent
> of the clk they're registering. We have of_clk_parent_fill() but that
> mostly only works for single clks registered from a DT node, which isn't
> the norm. Let's simplify this all by introducing two new ways of
> specifying clk parents.
> 
> The first method is an array of pointers to clk_hw structures
> corresponding to the parents at that index. This works for clks that are
> registered when we have access to all the clk_hw pointers for the
> parents.
> 
> The second method is a mix of clk_hw pointers and strings of local and
> global parent clk names. If the .fw_name member of the map is set we'll
> look for that clk by performing a DT based lookup of the device the clk
> is registered with and the .name specified in the map. If that fails,
> we'll fallback to the .name member and perform a global clk name lookup
> like we've always done before.
> 
> Using either one of these new methods is entirely optional. Existing
> drivers will continue to work, and they can migrate to this new approach
> as they see fit. Eventually, we'll want to get rid of the 'parent_names'
> array in struct clk_init_data and use one of these new methods instead.
> 
> Cc: Miquel Raynal <miquel.raynal@...tlin.com>
> Cc: Jerome Brunet <jbrunet@...libre.com>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Michael Turquette <mturquette@...libre.com>
> Cc: Jeffrey Hugo <jhugo@...eaurora.org>
> Cc: Chen-Yu Tsai <wens@...e.org>
> Cc: Rob Herring <robh@...nel.org>
> Signed-off-by: Stephen Boyd <sboyd@...nel.org>
> ---

Applied to clk-next