linux-kernel - Re: [PATCH v4] clk: qcom: rcg2: Cache CFG register updates for parked RCGs

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Date:   Tue, 17 May 2022 01:11:52 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Michael Turquette <mturquette@...libre.com>,
        Taniya Das <quic_tdas@...cinc.com>
Cc:     linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] clk: qcom: rcg2: Cache CFG register updates for parked RCGs

Quoting Bjorn Andersson (2022-04-26 14:21:36)
> As GDSCs are turned on and off some associated clocks are momentarily
> enabled for house keeping purposes. For this, and similar, purposes the
> "shared RCGs" will park the RCG on a source clock which is known to be
> available.
> When the RCG is parked, a safe clock source will be selected and
> committed, then the original source would be written back and upon enable
> the change back to the unparked source would be committed.
> 
> But starting with SM8350 this fails, as the value in CFG is committed by
> the GDSC handshake and without a ticking parent the GDSC enablement will
> time out.
> 
> This becomes a concrete problem if the runtime supended state of a
> device includes disabling such rcg's parent clock. As the device
> attempts to power up the domain again the rcg will fail to enable and
> hence the GDSC enablement will fail, preventing the device from
> returning from the suspended state.
> 
> This can be seen in e.g. the display stack during probe on SM8350.
> 
> To avoid this problem, the software needs to ensure that the RCG is
> configured to a active parent clock while it is disabled. This is done
> by caching the CFG register content while the shared RCG is parked on
> this safe source.
> 
> Writes to M, N and D registers are committed as they are requested. New
> helpers for get_parent() and recalc_rate() are extracted from their
> previous implementations and __clk_rcg2_configure() is modified to allow
> it to operate on the cached value.
> 
> Fixes: 7ef6f11887bd ("clk: qcom: Configure the RCGs to a safe source as needed")
> Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> ---

Reviewed-by: Stephen Boyd <sboyd@...nel.org>