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

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Message-ID: <87364ee8-4cde-49f5-949c-b01674798c37@tecnico.ulisboa.pt>
Date: Tue, 13 Jan 2026 13:59:39 +0000
From: Diogo Ivo <diogo.ivo@...nico.ulisboa.pt>
To: Jon Hunter <jonathanh@...dia.com>, 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 1/13/26 11:44, Jon Hunter wrote:
> 
> On 13/01/2026 11:35, 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 was tested on a Tegra210 platform (Smaug). However, due to the 
>>> similar
>>> approach in Tegra186 it is likely that not only this problem exists 
>>> there
>>> but that this patch also fixes it.
>>>
>>> Signed-off-by: Diogo Ivo <diogo.ivo@...nico.ulisboa.pt>
>>> ---
>>>   drivers/phy/tegra/xusb.c            | 23 +++++++++++++++++++++++
>>>   drivers/usb/gadget/udc/tegra-xudc.c |  4 ++++
>>>   drivers/usb/host/xhci-tegra.c       | 15 ++++++++++-----
>>>   include/linux/phy/tegra/xusb.h      |  1 +
>>>   4 files changed, 38 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/phy/tegra/xusb.c b/drivers/phy/tegra/xusb.c
>>> index c89df95aa6ca..e05c3f2d1421 100644
>>> --- a/drivers/phy/tegra/xusb.c
>>> +++ b/drivers/phy/tegra/xusb.c
>>> @@ -740,6 +740,29 @@ static void 
>>> tegra_xusb_parse_usb_role_default_mode(struct tegra_xusb_port *port)
>>>       }
>>>   }
>>> +bool tegra_xusb_usb2_port_wait_role_none(struct tegra_xusb_padctl 
>>> *padctl, int index)
>>> +{
>>> +    struct tegra_xusb_usb2_port *usb2 = 
>>> tegra_xusb_find_usb2_port(padctl,
>>> +                                      index);
>>> +    int retries = 5;
>>> +
>>> +    if (!usb2) {
>>> +        dev_err(&usb2->base.dev, "no port found for USB2 lane %u\n", 
>>> index);
>>> +        return false;
>>> +    }
>>> +
>>> +    do {
>>> +        if (usb2->role == USB_ROLE_NONE)
>>> +            return true;
>>> +
>>> +        usleep_range(50, 60);
>>> +    } while (retries--);
>>> +
>>> +    dev_err(&usb2->base.dev, "timed out waiting for USB_ROLE_NONE");
>>> +
>>> +    return false;
>>> +}
>>
>>
>> This patch is causing the following build error today with -next ...
> 
> Sorry this is not in -next, I had just applied locally on top!
> 
>>    MODPOST Module.symvers
>> ERROR: modpost: "tegra_xusb_usb2_port_wait_role_none" [drivers/usb/ 
>> host/ xhci-tegra.ko] undefined!
>>
>> The above function symbol needs to be exported.

You're right, thanks for catching this. I'll fix it in v2.

> 
> Nonetheless this needs to be fixed.
> 
> Jon
>