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

[Diogo Ivo <diogo.ivo@tecnico.ulisboa.pt>]

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?

Thanks,
Diogo

> Thanks
> Jon

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