linux-kernel - Re: [PATCH 3/5] phy: tegra: xusb: Fix ordering issue when switching roles on USB2 ports

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Message-ID: <dc383e93-0040-419d-9d3b-15eb15df9dbf@nvidia.com>
Date: Mon, 19 Jan 2026 14:31:51 +0000
From: Jon Hunter <jonathanh@...dia.com>
To: Diogo Ivo <diogo.ivo@...nico.ulisboa.pt>,
 Mathias Nyman <mathias.nyman@...el.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Thierry Reding <thierry.reding@...il.com>, JC Kuo <jckuo@...dia.com>,
 Vinod Koul <vkoul@...nel.org>, Kishon Vijay Abraham I <kishon@...nel.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>
Cc: linux-usb@...r.kernel.org, linux-tegra@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-phy@...ts.infradead.org,
 devicetree@...r.kernel.org
Subject: Re: [PATCH 3/5] phy: tegra: xusb: Fix ordering issue when switching
 roles on USB2 ports


On 15/01/2026 11:06, Diogo Ivo wrote:
> Hi Jonathan,
> 
> On 1/13/26 11:56, Jon Hunter wrote:
>>
>> On 04/12/2025 21:27, Diogo Ivo wrote:
>>> The current implementation of USB2 role switching on Tegra relies on
>>> whichever the previous USB controller driver was using the PHY to first
>>> "yield" it back to USB_ROLE_NONE before the next controller configures
>>> it for the new role. However, no mechanism to guarantee this ordering
>>> was implemented, and currently, in the general case, the configuration
>>> functions tegra_xhci_id_work() and tegra_xudc_usb_role_sw_work() end up
>>> running in the same order regardless of the transition being HOST- 
>>> >DEVICE
>>> or DEVICE->HOST, leading to one of these transitions ending up in a
>>> non-working state due to the new configuration being clobbered by the
>>> previous controller driver setting USB_ROLE_NONE after the fact.
>>>
>>> Fix this by introducing a helper that waits for the USB2 port’s current
>>> role to become USB_ROLE_NONE and add it in the configuration functions
>>> above before setting the role to either USB_ROLE_HOST or
>>> USB_ROLE_DEVICE. The specific parameters of the helper function are
>>> choices that seem reasonable in my testing and have no other basis.
>>
>> This is no information here about why 6 * 50/60us is deemed to be 
>> sufficient? May be it is, but a comment would be nice.
> 
> I missed this review comment and I'm not sure what you mean here. Do you
> want me to comment on the commit message on how I chose these
> parameters? If so it's as stated in the current message, I simply tested
> with these parameters and it worked and I really have no better basis
> for choosing them. If you mean adding a comment in the code I can do
> that for v2.

Yes please be explicit about how you arrived at these numbers. Ie. based 
upon your testing on what platform, etc.

Thanks
Jon

-- 
nvpublic