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Date: Tue, 5 Mar 2024 12:17:19 +0100
From: Markus Schneider-Pargmann <msp@baylibre.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Cc: Rob Herring <robh@kernel.org>, Nishanth Menon <nm@ti.com>, 
	Vignesh Raghavendra <vigneshr@ti.com>, Tero Kristo <kristo@kernel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>, Conor Dooley <conor+dt@kernel.org>, 
	Srinivas Kandagatla <srinivas.kandagatla@linaro.org>, Santosh Shilimkar <ssantosh@kernel.org>, 
	Andrew Davis <afd@ti.com>, linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org, 
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] dt-bindings: hwinfo: ti,k3-socinfo: Add nvmem-cells
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In-Reply-To: <48902771-5d3b-448a-8a74-ac18fb4f1a86@linaro.org>

Hi Krzysztof,

On Tue, Mar 05, 2024 at 08:43:03AM +0100, Krzysztof Kozlowski wrote:
> On 04/03/2024 11:36, Markus Schneider-Pargmann wrote:
> > Hi,
> > 
> > On Sat, Feb 17, 2024 at 03:25:30PM +0100, Krzysztof Kozlowski wrote:
> >> On 14/02/2024 10:31, Markus Schneider-Pargmann wrote:
> >>> Hi Rob,
> >>>
> >>> On Tue, Feb 06, 2024 at 06:43:05PM +0000, Rob Herring wrote:
> >>>> On Tue, Feb 06, 2024 at 03:37:09PM +0100, Markus Schneider-Pargmann wrote:
> >>>>> The information k3-socinfo requires is stored in an efuse area. This
> >>>>> area is required by other devices/drivers as well, so using nvmem-cells
> >>>>> can be a cleaner way to describe which information are used.
> >>>>>
> >>>>> If nvmem-cells are supplied, the address range is not required.
> >>>>> Cells chipvariant, chippartno and chipmanufacturer are introduced to
> >>>>> cover all required information.
> >>>>>
> >>>>> Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
> >>>>> Reviewed-by: Andrew Davis <afd@ti.com>
> >>>>> ---
> >>>>>  .../bindings/hwinfo/ti,k3-socinfo.yaml        | 23 ++++++++++++++++++-
> >>>>>  1 file changed, 22 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/Documentation/devicetree/bindings/hwinfo/ti,k3-socinfo.yaml b/Documentation/devicetree/bindings/hwinfo/ti,k3-socinfo.yaml
> >>>>> index dada28b47ea0..f085b7275b7d 100644
> >>>>> --- a/Documentation/devicetree/bindings/hwinfo/ti,k3-socinfo.yaml
> >>>>> +++ b/Documentation/devicetree/bindings/hwinfo/ti,k3-socinfo.yaml
> >>>>> @@ -26,9 +26,24 @@ properties:
> >>>>>    reg:
> >>>>>      maxItems: 1
> >>>>>  
> >>>>> +  nvmem-cells:
> >>>>> +    $ref: /schemas/types.yaml#/definitions/phandle-array
> >>>>> +
> >>>>> +  nvmem-cell-names:
> >>>>> +    items:
> >>>>> +      - const: chipvariant
> >>>>> +      - const: chippartno
> >>>>> +      - const: chipmanufacturer
> >>>>> +
> >>>>>  required:
> >>>>>    - compatible
> >>>>> -  - reg
> >>>>> +
> >>>>> +oneOf:
> >>>>> +  - required:
> >>>>> +      - reg
> >>>>> +  - required:
> >>>>> +      - nvmem-cells
> >>>>> +      - nvmem-cell-names
> >>>>>  
> >>>>>  additionalProperties: false
> >>>>>  
> >>>>> @@ -38,3 +53,9 @@ examples:
> >>>>>          compatible = "ti,am654-chipid";
> >>>>>          reg = <0x43000014 0x4>;
> >>>>>      };
> >>>>> +  - |
> >>>>> +    chipid: chipid@14 {
> >>>>> +        compatible = "ti,am654-chipid";
> >>>>
> >>>> This isn't compatible if you have a completely different way to access 
> >>>> it. 
> >>>
> >>> Thanks, it is not entirely clear to me how I could go forward with this?
> >>> Are you suggesting to use a different compatible? Or is it something
> >>> else I could do to proceed with this conversion?
> >>
> >> What you claim now, is that you have one device with entirely different
> >> interfaces and programming model. So either this is not the same device
> >> or you just wrote bindings to whatever you have in driver.
> >>
> >> Nothing in commit msg explains this.
> >>
> >> What you should do? Depends. If you just write bindings for driver, then
> >> stop. It's a NAK. Instead write bindings for hardware.
> >>
> >> If the first choice, just the hardware is somehow like this, then
> >> explain in commit msg and device description, how this device can be
> >> connected over other bus, not MMIO. You can draw some schematics in
> >> commit msg explaining architecture etc.
> > 
> > Sorry the information provided in the commit message is not very clear.
> > 
> > The basic access to the registes is still MMIO. nvmem is used to have a
> > better abstraction and cleaner description of the hardware.
> > 
> > Currently most of the data is exported using the parent syscon device.
> > The relevant data is read-only and contained in a single register with
> > offset 0x14:
> >   - Chip variant
> >   - Chip part number
> >   - Chip manufacturer
> > 
> > There are more read-only registers in this section of address space.
> > These are relevant to other components as they define the operating
> > points for example. For the OPP table relevant are chip variant and chip
> > speed (which is in a different register).
> > 
> > Instead of devices refering to this whole register range of 0x20000 in
> 
> Whaaaaat?
> 
> > size, I would like to introduce this nvmem abstraction in between that
> > describes the information and can directly be referenced by the devices
> > that depend on it. In this case the above mentioned register with offset
> > 0x14 is instead described as nvmem-layout like this:
> > 
> > 	nvmem-layout {
> > 		compatible = "fixed-layout";
> > 		#address-cells = <1>;
> > 		#size-cells = <1>;
> > 
> > 		chip_manufacturer: jtagidmfg@14 {
> > 			reg = <0x14 0x2>;
> > 			bits = <1 11>;
> > 		};
> > 
> > 		chip_partno: jtagidpartno@15 {
> > 			reg = <0x15 0x3>;
> > 			bits = <4 16>;
> > 		};
> > 
> > 		chip_variant: jtagidvariant@17 {
> > 			reg = <0x17 0x1>;
> > 			bits = <4 4>;
> > 		};
> > 
> > 		chip_speed: jtaguseridspeed@18 {
> > 			reg = <0x18 0x4>;
> > 			bits = <6 5>;
> > 		};
> > 
> > The underlying registers are still the same but they are not hidden
> > by the syscon phandles anymore.
> > 
> > The device that consumes this data would now use
> > 
> > 	nvmem-cells = <&chip_variant>, <&chip_speed>;
> > 	nvmem-cell-names = "chipvariant", "chipspeed";
> > 
> > instead of
> > 
> > 	syscon = <&wkup_conf>;
> 
> syscon allows you this as well - via phandle arguments.
> 
> nvmem is for non-volatile memory, like OCOTP and eFUSE. This is not for
> accessing regular MMIO registers of system-controller, regardless
> whether they are read-only or not (regmap handles this nicely, BTW).
> Although probably Apple efuses and few others can confuse here. It still
> looks like you convert regular system-controller block into nvmem,
> because you prefer that Linux driver abstraction...

The above mentioned data is set in the factory. There is other
non-volatile data, like device feature registers, in the same address
region, as well as OTP data like MAC and USB IDs. But it is not a pure
non-volatile memory region. The data is copied into these registers by
the ROM at boot.

Best,
Markus