Re: [PATCH 1/8] dt-bindings: phy: tegra-xusb: Add support for Tegra264

From: Wayne Chang <waynec@nvidia.com>
To: Conor Dooley <conor@kernel.org>
Cc: "mathias.nyman@intel.com" <mathias.nyman@intel.com>,
	"vkoul@kernel.org" <vkoul@kernel.org>,
	"neil.armstrong@linaro.org" <neil.armstrong@linaro.org>,
	"robh@kernel.org" <robh@kernel.org>,
	"krzk+dt@kernel.org" <krzk+dt@kernel.org>,
	"conor+dt@kernel.org" <conor+dt@kernel.org>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"thierry.reding@gmail.com" <thierry.reding@gmail.com>,
	Jon Hunter <jonathanh@nvidia.com>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
	"linux-tegra@vger.kernel.org" <linux-tegra@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-phy@lists.infradead.org" <linux-phy@lists.infradead.org>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>
Subject: Re: [PATCH 1/8] dt-bindings: phy: tegra-xusb: Add support for Tegra264
Date: Mon, 31 Aug 2026 18:28:01 +0000	[thread overview]
Message-ID: <56187858-3096-4db9-8d1a-6fb7e72f2eb3@nvidia.com> (raw)
In-Reply-To: <20260629-baffling-gag-05e20c9d925b@spud>

Hi Conor,

Thanks for the review and sorry for the late reply.

A fallback is not suitable here because Tegra264 XUSB padctl is not
programming-compatible with Tegra234 (or Tegra194), even though the
DT property set is the same.

The driver selects per-SoC data from the compatible string. Tegra264
differs from Tegra234 in at least:

- Tegra264 uses hardware tracking mode (trk_hw_mode), while Tegra234 
disables it.
- On Tegra234, CYA_TRK_CODE_UPDATE_ON_IDLE needs to be cleared so that 
the calibration
values can be captured while the controller is still in reset. This is 
not required on
Tegra264, where the corresponding handling is not even present.

A fallback of "nvidia,tegra234-xusb-padctl" would let an OS that only
knows Tegra234 probe Tegra264 with the wrong programming sequence.

That is also why Tegra234 did not fall back to Tegra194 in this same
binding: the DT schema is shared, but each generation needs its own
compatible.

Thanks,
Wayne

On 6/29/26 23:19, Conor Dooley wrote:
> On Mon, Jun 29, 2026 at 05:33:59PM +0800, Wayne Chang wrote:
>> Add the compatible string for the Tegra264 XUSB PHY.
>>
>> Signed-off-by: Wayne Chang<waynec@nvidia.com>
> Why is a fallback compatible not suitable here?
>
>
> Thanks,
> Conor.
>
>> ---
>>   .../devicetree/bindings/phy/nvidia,tegra194-xusb-padctl.yaml     | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/Documentation/devicetree/bindings/phy/nvidia,tegra194-xusb-padctl.yaml b/Documentation/devicetree/bindings/phy/nvidia,tegra194-xusb-padctl.yaml
>> index d8de900a4fce..f6fe3c90b5b0 100644
>> --- a/Documentation/devicetree/bindings/phy/nvidia,tegra194-xusb-padctl.yaml
>> +++ b/Documentation/devicetree/bindings/phy/nvidia,tegra194-xusb-padctl.yaml
>> @@ -45,6 +45,7 @@ properties:
>>       enum:
>>         - nvidia,tegra194-xusb-padctl
>>         - nvidia,tegra234-xusb-padctl
>> +      - nvidia,tegra264-xusb-padctl
>>   
>>     reg:
>>       items:
>> -- 
>> 2.25.1
>>

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