linux-kernel - Re: [PATCH v1 1/2] scsi: dt-bindings: ufs: Add vcc-voltage-level for UFS

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Date:   Wed, 23 Sep 2020 10:32:44 -0700
From:   nguyenb@...eaurora.org
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Rob Herring <robh@...nel.org>, Can Guo <cang@...eaurora.org>,
        Asutosh Das <asutoshd@...eaurora.org>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        SCSI <linux-scsi@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Avri Altman <Avri.Altman@....com>,
        Vinod Koul <vkoul@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/2] scsi: dt-bindings: ufs: Add vcc-voltage-level for
 UFS

On 2020-09-21 17:36, Bjorn Andersson wrote:
> On Mon 21 Sep 19:22 CDT 2020, nguyenb@...eaurora.org wrote:
> 
>> On 2020-09-18 12:01, Rob Herring wrote:
>> > On Tue, Sep 15, 2020 at 2:10 AM <nguyenb@...eaurora.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@...eaurora.org>
>> > > >> Signed-off-by: Asutosh Das <asutoshd@...eaurora.org>
>> > > >> Signed-off-by: Bao D. Nguyen <nguyenb@...eaurora.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.
>> 
> 
> It sounds like you're suggesting that we detect the UFS device using
> some voltage, then depending on version we might lower it to 2.5V.
Yes, that is one possible solution.

> I'm afraid I don't see any of this either documented or implemented in
> these patches.
I was responding to a comment about detecting the device version and 
change the voltage
based on the detection. It is not implemented in this patch. Maybe I 
should stop
discussing another possible solution, even though related to the topic, 
it is not
implemented in this patch.

> 
> What is this initial detection voltage and how to you configure it?
The initial voltage would be 2.9V and is lowered to 2.5V if UFS3.0+ 
device is detected.
We would call the regulator_set_voltage() to set to a specific voltage 
level.

Regards,
Bao

> 
> Regards,
> Bjorn
> 
>> >
>> > > 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