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From:   "Naveen Kumar Goud Arepalli (QUIC)" <quic_narepall@quicinc.com>
To:     Andrew Halaney <ahalaney@redhat.com>
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Subject: RE: [PATCH V1] arm64: dts: qcom: sa8775p-ride: Remove min and max
 voltages for L8A
Thread-Topic: [PATCH V1] arm64: dts: qcom: sa8775p-ride: Remove min and max
 voltages for L8A
Thread-Index: AQHZvt7QYX0nSDdHt02LA98zFub0Fa/Le8CAgACAp6A=
Date:   Wed, 26 Jul 2023 12:32:12 +0000
Message-ID: <516a54da44724001895f7e50634ad884@quicinc.com>
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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=20
> configurations for UFS devices.
> -UFS 2.x devices: 2.70V - 3.60V
> -UFS 3.x devices: 2.40V - 2.70V
>=20
> As sa8775p-ride supports both ufs 2.x and ufs 3.x devices, remove=20
> min/max voltages for L8A regulator. Initial voltage of L8A will be set=20
> to 2.504v or 2.952v during PON depending on the UFS device type. On=20
> sa8775, UFS is the only client in Linux for L8A and this regulator=20
> will be voted only for enabling/disabling.
>=20
> Signed-off-by: Naveen Kumar Goud Arepalli <quic_narepall@quicinc.com>
> ---
>  arch/arm64/boot/dts/qcom/sa8775p-ride.dts | 2 --
>  1 file changed, 2 deletions(-)
>=20
> diff --git a/arch/arm64/boot/dts/qcom/sa8775p-ride.dts=20
> 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 @@
> =20
>  		vreg_l8a: ldo8 {
>  			regulator-name =3D "vreg_l8a";
> -			regulator-min-microvolt =3D <2504000>;
> -			regulator-max-microvolt =3D <3300000>;
>  			regulator-initial-mode =3D <RPMH_REGULATOR_MODE_HPM>;
>  			regulator-allow-set-load;
>  			regulator-allowed-modes =3D <RPMH_REGULATOR_MODE_LPM
> --
> 2.17.1
>=20

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?:
>>> Yes, we are removing the range of the regulator.

    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?
       >>>> For a UFS device 3.x, we cannot start as UFS 2.0. vcc has to be=
 as per UFS 3.x recommendations.

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.
>>>> As per upstream UFS driver,  voltage voting is not there and we vote o=
nly for enable/disable .=20
Since UFS is the only client in Linux for this rail (L8A ), we don't need m=
in and max range to support
UFS 2.x and 3.x cards.

Thanks,
Naveen