Re: [PATCH V2 4/6] regulator: qcom_smd: Add support to define the bootup voltage

From: Devi Priya <quic_devipriy@quicinc.com>
To: Mark Brown <broonie@kernel.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>
Cc: <agross@kernel.org>, <andersson@kernel.org>,
	<lgirdwood@gmail.com>, <robh+dt@kernel.org>,
	<krzysztof.kozlowski+dt@linaro.org>,
	<linux-arm-msm@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<devicetree@vger.kernel.org>, <quic_srichara@quicinc.com>,
	<quic_gokulsri@quicinc.com>, <quic_sjaganat@quicinc.com>,
	<quic_kathirav@quicinc.com>, <quic_arajkuma@quicinc.com>,
	<quic_anusha@quicinc.com>, <quic_ipkumar@quicinc.com>
Subject: Re: [PATCH V2 4/6] regulator: qcom_smd: Add support to define the bootup voltage
Date: Fri, 3 Mar 2023 18:51:49 +0530	[thread overview]
Message-ID: <39f73580-f263-de0e-6819-89c3f4c75c3a@quicinc.com> (raw)
In-Reply-To: <Y/aeu5ua7cY5cGON@sirena.org.uk>

On 2/23/2023 4:31 AM, Mark Brown wrote:
> On Wed, Feb 22, 2023 at 11:11:42PM +0100, Konrad Dybcio wrote:
> 
>> Thinking about it again, this seems like something that could be
>> generalized and introduced into regulator core.. Hardcoding this
>> will not end well.. Not to mention it'll affect all mp5496-using
>> boards that are already upstream.
> 
>> WDYT about regulator-init-microvolts Mark?
> 
> The overwhelming majority of devices that have variable voltages
> support readback, these Qualcomm firmware devices are pretty much
> unique in this regard.  We don't want a general property to set a
> specific voltage since normally we should be using the
> constraints and don't normally need to adjust things immediately
> since we can tell what the current voltage is.
> 
> This is pretty much just going to be a device specific bodge,
> ideally something that does know what the voltage is would be
> able to tell us at runtime but if that's not possible then
> there's no good options.  If the initial voltage might vary based
> on board then a device specific DT property might be less
> terrible, if it's determined by the regulator the current code
> seems fine.  Or just leave the current behavour, if the
> constraints are accurate then hopefully a temporary dip in
> voltage is just inelegant rather than an issue.  Indeed the
> current behaviour might well save power if you've got a voltage
> range configured and nothing actually ever gets round to setting
> the voltage (which is depressingly common, people seem keen on
> setting voltage ranges even when the voltage is never varied in
> practice).

Hi Mark, The initial bootup voltage is actually blown into the OTP 
register of the PMIC and it remains the same across boards for IPQ9574 
SoC. Initially the SoC runs at 800MHz with a voltage of 875mV set by the 
bootloaders. As kernel does not know the initial voltage, during 
regulator registration the framework considers the current voltage to be 
zero and tries to bring up the regulator to minimum supported voltage of 
600mV. This causes the dip which might be of concern in SS parts where 
the voltage might be insufficient leading to silent reboots.

Best Regards,
Devi Priya