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

[nguyenb]

On 2020-09-18 12:01, Rob Herring wrote:
> On Tue, Sep 15, 2020 at 2:10 AM <nguyenb@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > >> Signed-off-by: Asutosh Das <asutoshd@xxxxxxxxxxxxxx>
> > >> Signed-off-by: Bao D. Nguyen <nguyenb@xxxxxxxxxxxxxx>
> > >> ---
> > >>  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