linux-kernel - Re: [PATCH V1] arm64: dts: qcom: sa8775p-ride: Remove min and max voltages for L8A

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Message-ID: <a3l7356miuuapf5dakgfchdjmxjp62ynvle4ta3hejd3tjvzd4@e2t2zm6jh7hb>
Date:   Tue, 25 Jul 2023 16:50:49 -0500
From:   Andrew Halaney <ahalaney@...hat.com>
To:     Naveen Kumar Goud Arepalli <quic_narepall@...cinc.com>
Cc:     quic_nitirawa@...cinc.com, agross@...nel.org, andersson@...nel.org,
        konrad.dybcio@...aro.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, conor+dt@...nel.org,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V1] arm64: dts: qcom: sa8775p-ride: Remove min and max
 voltages for L8A

On Tue, Jul 25, 2023 at 03:30:07PM +0530, Naveen Kumar Goud Arepalli wrote:
> L8A is the supply for UFS VCC, UFS specification allows different VCC
> configurations for UFS devices.
> -UFS 2.x devices: 2.70V - 3.60V
> -UFS 3.x devices: 2.40V - 2.70V
> 
> As sa8775p-ride supports both ufs 2.x and ufs 3.x devices, remove min/max
> voltages for L8A regulator. Initial voltage of L8A will be set to 2.504v
> or 2.952v during PON depending on the UFS device type. On sa8775, UFS is
> the only client in Linux for L8A and this regulator will be voted only
> for enabling/disabling.
> 
> Signed-off-by: Naveen Kumar Goud Arepalli <quic_narepall@...cinc.com>
> ---
>  arch/arm64/boot/dts/qcom/sa8775p-ride.dts | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/arm64/boot/dts/qcom/sa8775p-ride.dts b/arch/arm64/boot/dts/qcom/sa8775p-ride.dts
> index ed76680410b4..6f3891a09e59 100644
> --- a/arch/arm64/boot/dts/qcom/sa8775p-ride.dts
> +++ b/arch/arm64/boot/dts/qcom/sa8775p-ride.dts
> @@ -98,8 +98,6 @@
>  
>  		vreg_l8a: ldo8 {
>  			regulator-name = "vreg_l8a";
> -			regulator-min-microvolt = <2504000>;
> -			regulator-max-microvolt = <3300000>;
>  			regulator-initial-mode = <RPMH_REGULATOR_MODE_HPM>;
>  			regulator-allow-set-load;
>  			regulator-allowed-modes = <RPMH_REGULATOR_MODE_LPM
> -- 
> 2.17.1
> 

Reviewing with very little expertise in the area....
A few questions below that would help me understand this a bit better.

Does it make sense to *not* set the range of the regulator at all?:

    1. A board dts knows its UFS device
    2. Is UFS backwards compatible with respect to UFS2/UFS3?
       I don't know how the version is determined, but if it's a
       "start at UFS2, go to UFS3" should it be scaled as that goes?

Relying on the bootloader to set up the device before the kernel starts
seems like a direction that should be actively avoided instead of
depended on in my opinion.

Thanks,
Andrew