linux-kernel - Re: [PATCH v6 1/6] tty: serial: qcom_geni_serial: Use OPP API to set clk/perf state

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Message-ID: <20200722052444.updchi2yfjgbf3hb@vireshk-mac-ubuntu>
Date:   Wed, 22 Jul 2020 10:54:44 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Rajendra Nayak <rnayak@...eaurora.org>, agross@...nel.org,
        bjorn.andersson@...aro.org, robdclark@...omium.org,
        robdclark@...il.com, stanimir.varbanov@...aro.org,
        mka@...omium.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Akash Asthana <akashast@...eaurora.org>,
        linux-serial@...r.kernel.org
Subject: Re: [PATCH v6 1/6] tty: serial: qcom_geni_serial: Use OPP API to set
 clk/perf state

On 21-07-20, 01:43, Stephen Boyd wrote:
> It seems that dev_pm_opp_set_rate() calls _find_opp_table() and finds
> something that isn't an error pointer but then dev_pm_opp_of_add_table()
> returns an error value because there isn't an operating-points property
> in DT. We're getting saved because this driver also happens to call
> dev_pm_opp_set_clkname() which allocates the OPP table a second time
> (because the first time it got freed when dev_pm_opp_of_add_table()
> return -ENODEV because the property was missing).
> 
> Why do we need 'has_opp_table' logic? It seems that we have to keep
> track of the fact that dev_pm_opp_of_add_table() failed so that we don't
> put the table again, but then dev_pm_opp_set_clkname() can be called
> to allocate the table regardless.
> 
> This maintainer is paying very close attention

:)

> to super confusing code like
> this:
> 
> 	if (drv->has_opp_table)
> 		dev_pm_opp_of_remove_table(dev);
> 	dev_pm_opp_put_clkname(drv->opp_table);
> 
> which reads as "if I have an opp table remove it and oh by the way
> remove the clk name for this opp table pointer I also happen to always
> have".
> 
> Maybe I would be happier if dev_pm_opp_of_table() went away and we just
> had dev_pm_opp_add_table(const struct opp_config *config) that did all
> the things for us like set a clk name, set the supported hw, set the
> prop name, etc. based on the single config struct pointer and also
> parsed out the OPP table from DT or just ignored that if there isn't any
> operating-points property. Then the caller wouldn't need to keep track
> of 'if has_opp_table' because it doesn't seem to actually care and the
> core is happy to allocate a table for the device anyway so long as it
> sets a clk name.

The config style wouldn't work as well as we don't really want to
allocate an OPP table if the property isn't found in DT.

All the mess is coming from the fact that I wanted to make it easy for
the generic drivers to have code which can do opp-set-rate or
clk-set-rate depending on how the platform is configured. While the
intention was fine, the end result is still not great as you figured
out.

Because we need to keep a flag to make the right decision anyway, I
wonder if doing this is the best solution we have at hand.

if (opp-table-present)
        opp_set_rate();
else
        clk_set_rate();

Or maybe stop printing errors from dev_pm_opp_of_remove_table() if the
OPP table isn't found. And so we can get rid of the flag.

-- 
viresh