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

[Jon Hunter <jonathanh@nvidia.com>]

On 13/01/2026 14:05, Diogo Ivo wrote:
> 
> 
> 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.
>>
>>> 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@tecnico.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);
>>
>> This appears to be a bug. If !usb2 then dereference usb2->base anyway.
> 
> It is a bug, will fix in v2.
> 
>>> +        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;
>>> +}
>>> +
>>>   static int tegra_xusb_usb2_port_parse_dt(struct 
>>> tegra_xusb_usb2_port *usb2)
>>>   {
>>>       struct tegra_xusb_port *port = &usb2->base;
>>> diff --git a/drivers/usb/gadget/udc/tegra-xudc.c b/drivers/usb/ 
>>> gadget/ udc/tegra-xudc.c
>>> index 0c38fc37b6e6..72d725659e5f 100644
>>> --- a/drivers/usb/gadget/udc/tegra-xudc.c
>>> +++ b/drivers/usb/gadget/udc/tegra-xudc.c
>>> @@ -698,8 +698,12 @@ static void tegra_xudc_restore_port_speed(struct 
>>> tegra_xudc *xudc)
>>>   static void tegra_xudc_device_mode_on(struct tegra_xudc *xudc)
>>>   {
>>> +    int port = tegra_xusb_padctl_get_port_number(xudc->curr_utmi_phy);
>>>       int err;
>>> +    if (!tegra_xusb_usb2_port_wait_role_none(xudc->padctl, port))
>>> +        return;
>>> +
>>>       pm_runtime_get_sync(xudc->dev);
>>>       tegra_phy_xusb_utmi_pad_power_on(xudc->curr_utmi_phy);
>>> diff --git a/drivers/usb/host/xhci-tegra.c b/drivers/usb/host/xhci- 
>>> tegra.c
>>> index 9c69fccdc6e8..9944593166a3 100644
>>> --- a/drivers/usb/host/xhci-tegra.c
>>> +++ b/drivers/usb/host/xhci-tegra.c
>>> @@ -1352,18 +1352,23 @@ static void tegra_xhci_id_work(struct 
>>> work_struct *work)
>>>       struct tegra_xusb_mbox_msg msg;
>>>       struct phy *phy = tegra_xusb_get_phy(tegra, "usb2",
>>>                               tegra->otg_usb2_port);
>>> +    enum usb_role role = USB_ROLE_NONE;
>>>       u32 status;
>>>       int ret;
>>>       dev_dbg(tegra->dev, "host mode %s\n", str_on_off(tegra- 
>>> >host_mode));
>>> -    mutex_lock(&tegra->lock);
>>
>> Extra blank line here.
> 
> Will fix in v2.
> 
>>> -    if (tegra->host_mode)
>>> -        phy_set_mode_ext(phy, PHY_MODE_USB_OTG, USB_ROLE_HOST);
>>> -    else
>>> -        phy_set_mode_ext(phy, PHY_MODE_USB_OTG, USB_ROLE_NONE);
>>> +    if (tegra->host_mode) {
>>> +        if (!tegra_xusb_usb2_port_wait_role_none(tegra->padctl,
>>> +                             tegra->otg_usb2_port))
>>> +            return;
>>> +        role = USB_ROLE_HOST;
>>> +    }
>>> +
>>> +    mutex_lock(&tegra->lock);
>>> +    phy_set_mode_ext(phy, PHY_MODE_USB_OTG, role);
>>>       mutex_unlock(&tegra->lock);
>>
>> I am trying to understand why you opted to implement it this way 
>> around and not add the wait loop after setting to the mode to 
>> USB_ROLE_NONE in the original code all within the context of the mutex?
> 
> I did that to minimize the amount of time we wait while holding the
> mutex, as we can now possibly wait a significant amount of time for the
> role switch. Is this an unneccessary optimization?

Do you mean it will be longer than a few 100us?

Jon

-- 
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