Re: [PATCH] usb: cdns3: exit cdns3_gadget_udc_start if FAST_REG_ACCESS cannot be set

[Siddharth Vadapalli <s-vadapalli@xxxxxx>]

On Fri, Feb 07, 2025 at 10:25:23AM +0800, Peter Chen wrote:

Hello Peter,

> On 25-02-06 18:29:36, Siddharth Vadapalli wrote:
> > When the device is in a low power state, access to the following
> > registers takes a long time:
> > - EP_CFG
> > - EP_TRADDR
> > - EP_CMD
> > - EP_SEL
> > - EP_STS
> > - USB_CONF
> > 
> > To address this, the fast register access feature can be enabled by
> > setting PUSB_PWR_FST_REG_ACCESS bit of the USB_PWR register, which
> > allows quick access by software. Software is expected to poll on
> > PUSB_PWR_FST_REG_ACCESS_STAT to ensure that fast register access has
> > been enabled by the controller. Attempting to access any of the
> > aforementioned registers after setting PUSB_PWR_FST_REG_ACCESS but
> > before PUSB_PWR_FST_REG_ACCESS_STAT has been set will result in
> > undefined behavior and potentially result in system hang.
> > 
> > Hence, poll on PUSB_PWR_FST_REG_ACCESS_STAT before proceeding with
> > gadget configuration, and exit if it cannot be enabled.
> > 
> > Fixes: b5148d946f45 ("usb: cdns3: gadget: set fast access bit")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Siddharth Vadapalli <s-vadapalli@xxxxxx>
> > ---
> > 
> > Hello,
> > 
> > This patch is based on commit
> > 92514ef226f5 Merge tag 'for-6.14-rc1-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux
> > of Mainline Linux.
> > 
> > Regards,
> > Siddharth.
> > 
> >  drivers/usb/cdns3/cdns3-gadget.c | 18 ++++++++++++++++--
> >  1 file changed, 16 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/usb/cdns3/cdns3-gadget.c b/drivers/usb/cdns3/cdns3-gadget.c
> > index fd1beb10bba7..b62691944272 100644
> > --- a/drivers/usb/cdns3/cdns3-gadget.c
> > +++ b/drivers/usb/cdns3/cdns3-gadget.c
> > @@ -2971,8 +2971,6 @@ static void cdns3_gadget_config(struct cdns3_device *priv_dev)
> >  	/* enable generic interrupt*/
> >  	writel(USB_IEN_INIT, &regs->usb_ien);
> >  	writel(USB_CONF_CLK2OFFDS | USB_CONF_L1DS, &regs->usb_conf);
> > -	/*  keep Fast Access bit */
> > -	writel(PUSB_PWR_FST_REG_ACCESS, &priv_dev->regs->usb_pwr);
> >  
> >  	cdns3_configure_dmult(priv_dev, NULL);
> >  }
> > @@ -2990,6 +2988,8 @@ static int cdns3_gadget_udc_start(struct usb_gadget *gadget,
> >  	struct cdns3_device *priv_dev = gadget_to_cdns3_device(gadget);
> >  	unsigned long flags;
> >  	enum usb_device_speed max_speed = driver->max_speed;
> > +	int ret;
> > +	u32 reg;
> >  
> >  	spin_lock_irqsave(&priv_dev->lock, flags);
> >  	priv_dev->gadget_driver = driver;
> > @@ -2997,6 +2997,20 @@ static int cdns3_gadget_udc_start(struct usb_gadget *gadget,
> >  	/* limit speed if necessary */
> >  	max_speed = min(driver->max_speed, gadget->max_speed);
> >  
> > +	/*  keep Fast Access bit */
> > +	writel(PUSB_PWR_FST_REG_ACCESS, &priv_dev->regs->usb_pwr);
> > +	reg = readl(&priv_dev->regs->usb_pwr);
> > +	if (!(reg & PUSB_PWR_FST_REG_ACCESS_STAT)) {
> > +		ret = readl_poll_timeout_atomic(&priv_dev->regs->usb_pwr, reg,
> > +						(reg & PUSB_PWR_FST_REG_ACCESS_STAT),
> > +						10, 1000);
> > +		if (ret) {
> > +			dev_err(priv_dev->dev, "Failed to enable fast access\n");
> > +			spin_unlock_irqrestore(&priv_dev->lock, flags);
> > +			return ret;
> > +		}
> > +	}
> > +
> >  	switch (max_speed) {
> >  	case USB_SPEED_FULL:
> >  		writel(USB_CONF_SFORCE_FS, &priv_dev->regs->usb_conf);
> 
> Hi Siddharth,
> 
> Would you please keep this change at cdns3_gadget_config in case the
> controller is power lost during the system suspend?

I did think of that initially, but the problem with doing so is that we
are already accessing USB_CONF above in the "switch(max_speed)" section.
The PUSB_PWR_FST_REG_ACCESS bit needs to be set before accessing any of:
- EP_CFG
- EP_TRADDR
- EP_CMD
- EP_SEL
- EP_STS
- USB_CONF

Please let me know if you have an alternate suggestion to address the
above.

Regards,
Siddharth.