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Date:   Tue, 15 Sep 2020 01:10:20 -0700
From:   nguyenb@codeaurora.org
To:     Rob Herring <robh@kernel.org>
Cc:     cang@codeaurora.org, asutoshd@codeaurora.org,
        martin.petersen@oracle.com, linux-scsi@vger.kernel.org,
        linux-arm-msm@vger.kernel.org,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Avri Altman <Avri.Altman@wdc.com>,
        Vinod Koul <vkoul@kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v1 1/2] scsi: dt-bindings: ufs: Add vcc-voltage-level for
 UFS
In-Reply-To: <20200914183505.GA357@bogus>
References: <cover.1598939393.git.nguyenb@codeaurora.org>
 <0a9d395dc38433501f9652a9236856d0ac840b77.1598939393.git.nguyenb@codeaurora.org>
 <20200914183505.GA357@bogus>
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On 2020-09-14 11:35, Rob Herring wrote:
> On Mon, Aug 31, 2020 at 11:00:47PM -0700, Bao D. Nguyen wrote:
>> UFS's specifications supports a range of Vcc operating
>> voltage levels. Add documentation for the UFS's Vcc voltage
>> levels setting.
>> 
>> Signed-off-by: Can Guo <cang@codeaurora.org>
>> Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
>> Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>
>> ---
>>  Documentation/devicetree/bindings/ufs/ufshcd-pltfrm.txt | 2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/Documentation/devicetree/bindings/ufs/ufshcd-pltfrm.txt 
>> b/Documentation/devicetree/bindings/ufs/ufshcd-pltfrm.txt
>> index 415ccdd..7257b32 100644
>> --- a/Documentation/devicetree/bindings/ufs/ufshcd-pltfrm.txt
>> +++ b/Documentation/devicetree/bindings/ufs/ufshcd-pltfrm.txt
>> @@ -23,6 +23,8 @@ Optional properties:
>>                            with "phys" attribute, provides phandle to 
>> UFS PHY node
>>  - vdd-hba-supply        : phandle to UFS host controller supply 
>> regulator node
>>  - vcc-supply            : phandle to VCC supply regulator node
>> +- vcc-voltage-level     : specifies voltage levels for VCC supply.
>> +                          Should be specified in pairs (min, max), 
>> units uV.
> 
> The expectation is the regulator pointed to by 'vcc-supply' has the
> voltage constraints. Those constraints are supposed to be the board
> constraints, not the regulator operating design constraints. If that
> doesn't work for your case, then it should be addressed in a common way
> for the regulator binding.
The UFS regulator has a min_uV and max_uV limits. Currently, the min and 
max are hardcoded
to UFS2.1 Spec allowed values of 2.7V and 3.6V respectively.
With this change, I am trying to fix a couple issues:
1. The 2.7V min value only applies to UFS2.1 devices. with UFS3.0+ 
devices, the VCC min should be 2.4V.
Hardcoding the min_uV to 2.7V does not work for UFS3.0+ devices.

2. Allow users to select a different Vcc voltage within the allowed 
range.
Using the min value, the UFS device is operating at marginal Vcc 
voltage.
In addition the PMIC and the board designs may add some variables 
especially at extreme
temperatures. We observe stability issues when using the min Vcc 
voltage.

> 
> Also, properties with units must have a unit suffix.
Yes, I agree.
> 
> Rob