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Date:   Mon, 21 Sep 2020 17:22:15 -0700
From:   nguyenb@codeaurora.org
To:     Rob Herring <robh@kernel.org>
Cc:     Can Guo <cang@codeaurora.org>,
        Asutosh Das <asutoshd@codeaurora.org>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        SCSI <linux-scsi@vger.kernel.org>,
        linux-arm-msm <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: <CAL_Jsq+YV-GjAhVVHtgNz6xFR=bEgSwWKY+QGixRQJ5Ov75pag@mail.gmail.com>
References: <cover.1598939393.git.nguyenb@codeaurora.org>
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 <20200914183505.GA357@bogus>
 <d332e61cea4fef237507f1404efa724a@codeaurora.org>
 <CAL_Jsq+YV-GjAhVVHtgNz6xFR=bEgSwWKY+QGixRQJ5Ov75pag@mail.gmail.com>
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On 2020-09-18 12:01, Rob Herring wrote:
> On Tue, Sep 15, 2020 at 2:10 AM <nguyenb@codeaurora.org> wrote:
>> 
>> 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.
> 
> Don't you know the device version attached and can adjust the voltage
> based on that? Or you have to set the voltage first?
Yes it is one of the solutions. Once detect the UFS device is version 
3.0+, you can lower
the voltage to 2.5V from the hardcoded value used by the driver. 
However, to change the
Vcc voltage, the host needs to follow a sequence to ensure safe 
operations after Vcc change
(device has to be in sleep mode, Vcc needs to go down to 0 then up to 
2.5V.)
Also same sequence is repeated for every host initialization which is 
inconvenient.

> 
>> 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.
> 
> Again, we have standard regulator properties for this already that you
> can tune per board.
Thank you for the suggestion.

> 
> Rob