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Subject: Re: [PATCH v4] clk: qcom: rcg2: Cache CFG register updates for parked RCGs
From:   Stephen Boyd <sboyd@kernel.org>
Cc:     linux-arm-msm@vger.kernel.org, linux-clk@vger.kernel.org,
        linux-kernel@vger.kernel.org
To:     Bjorn Andersson <bjorn.andersson@linaro.org>,
        Michael Turquette <mturquette@baylibre.com>,
        Taniya Das <quic_tdas@quicinc.com>
Date:   Tue, 17 May 2022 01:11:52 -0700
User-Agent: alot/0.10
Message-Id: <20220517081154.AA72FC34118@smtp.kernel.org>
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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.
>=20
> 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.
>=20
> 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.
>=20
> This can be seen in e.g. the display stack during probe on SM8350.
>=20
> 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.
>=20
> 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.
>=20
> Fixes: 7ef6f11887bd ("clk: qcom: Configure the RCGs to a safe source as n=
eeded")
> Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> ---

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