linux-kernel - Re: [PATCH v1] clk: Probe defer clk

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Message-ID: <873279a6-0755-2094-ec99-d9e5f6c963dd@codeaurora.org>
Date:   Wed, 6 Mar 2019 14:48:13 -0700
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     mturquette@...libre.com, sboyd@...nel.org
Cc:     bjorn.andersson@...aro.org, georgi.djakov@...aro.org,
        linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] clk: Probe defer clk_get() on orphans

Ping?

Stephen, I know as this depends on your clock parent handling series 
(happens to apply just fine to v2), its not going to be accepted until 
that gets sorted out, but do you have any thoughts on if this seems like 
an appropriate thing to do, or if you'd like to see a different solution?

On 2/11/2019 11:57 AM, Jeffrey Hugo wrote:
> If a parent to a clock comes from outside that clock's provider, the parent
> may not be present at the time the clock is registered (ie the parent comes
> from another driver that has not yet probed).  The clock can still be
> registered, and a reference to it obtained, however that clock may not be
> fully functional - ie get_rate might return an invalid value.
> 
> This has been a problem that has resulted in the UART console breaking on
> some Qualcomm SoCs, as the UART baud rate is based on a clock that is the
> child of XO.  Due to the large chain of dependencies, its possible that the
> RPM has not provided XO by the time that the UART driver probes, gets the
> baud rate clock, and calls get_rate - which returns 0 and results in a bad
> configuration.
> 
> An orphan clock is a clock that is missing a parent or some other ancestor.
> Since the parent is defined, we can assume that it is expected to appear at
> some point in a properly configured system (all bets are off if a required
> driver is not compiled, etc), and it is unlikely that the clock can be
> properly consumed during the time the clock is an orphan.  Therefore,
> return EPROBE_DEFER for orphan clocks so that consumers wait until the
> parent chain is established, and proper clock operation can occur.
> 
> Signed-off-by: Jeffrey Hugo <jhugo@...eaurora.org>
> ---
> 
> This is based upon the "Rewrite clk parent handling" series at [1], and assumes
> that the suspected missing line commented on at [2] is added.
> 
> The idea for this solution came from [3] and [4].
> 
> [1] https://lore.kernel.org/lkml/20190129061021.94775-1-sboyd@kernel.org/T/#u
> [2] https://lkml.org/lkml/2019/2/11/1634
> [3] https://lkml.org/lkml/2019/2/6/382
> [4] https://lkml.org/lkml/2015/12/27/209
> 
>   drivers/clk/clk.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index 34e9524ea25d..cf71e0614282 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -3405,6 +3405,10 @@ struct clk *clk_hw_create_clk(struct device *dev, struct clk_hw *hw,
>   		return ERR_CAST(hw);
>   
>   	core = hw->core;
> +
> +	if (core->orphan)
> +		return ERR_PTR(-EPROBE_DEFER);
> +
>   	clk = alloc_clk(core, dev_id, con_id);
>   	if (IS_ERR(clk))
>   		return clk;
> 


-- 
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.