Re: [PATCH] usb: musb: Drop the D+ pullup during system suspend

[Andreas Kemnade <andreas@kemnade.info>]

On Sun, 27 Sep 2026 23:41:45 +0700
Nguyen Minh Tien <tien.nguyenminh@embeddedlinux.blog> wrote:

> When musb_suspend() clears DEVCTL, the host sees a disconnect. But VBUS
> is still there, so the controller can start a new session on its own
> and, with SOFTCONN still set, pull D+ up again while its interrupts are
> masked. The host then fails to enumerate the gadget ("unable to
> enumerate USB device") and gives up, and nothing at resume makes it try
> again. On a T113-S3 board the gadget link never survived an s2idle
> cycle.
> 
> Clear SOFTCONN once the context is saved; musb_restore_context() puts
> it back on resume. This is what the FIXME asked for, as USB can't wake
> us in time with the interrupts masked. On da8xx, which keeps the
> session over suspend, the gadget now disconnects too.
> 
> Fixes: 6fc6f4b87cb3 ("usb: musb: Disable interrupts on suspend, enable them on resume")
> Cc: stable@vger.kernel.org
> Signed-off-by: Nguyen Minh Tien <tien.nguyenminh@embeddedlinux.blog>
> ---
> I found this on a T113-S3 board (sunxi, s2idle, Intel xHCI host): ssh
> over the gadget never came back after a suspend. With the patch, all 30
> cycles I ran re-enumerated after resume.
> 
> To check for regressions, I also tried a BeagleBone Black (AM335x,
> dsps glue, suspend to RAM). There the link came back after every
> resume (10 cycles without the patch, 30 with it), probably because
> am335x_phy_suspend() powers the PHY off. 

Yes, other glue layers power off the phy, too. so this issue becomes
undiscovered. I think that difference should be commented in the code
so that everybody touching this is aware of the difference.

Regards,
Andreas
> 
>  drivers/usb/musb/musb_core.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/usb/musb/musb_core.c b/drivers/usb/musb/musb_core.c
> index 73ac25f536..272683f14d 100644
> --- a/drivers/usb/musb/musb_core.c
> +++ b/drivers/usb/musb/musb_core.c
> @@ -2825,18 +2825,24 @@ static int musb_suspend(struct device *dev)
>  
>  	spin_lock_irqsave(&musb->lock, flags);
>  
> +	musb_save_context(musb);
> +
>  	if (is_peripheral_active(musb)) {
> -		/* FIXME force disconnect unless we know USB will wake
> -		 * the system up quickly enough to respond ...
> +		/*
> +		 * We can't answer a host with the interrupts off, so drop the
> +		 * D+ pullup. musb_restore_context() puts back the state saved
> +		 * above.
>  		 */
> +		u8 power = musb_readb(musb->mregs, MUSB_POWER);
> +
> +		musb_writeb(musb->mregs, MUSB_POWER,
> +			    power & ~MUSB_POWER_SOFTCONN);
>  	} else if (is_host_active(musb)) {
>  		/* we know all the children are suspended; sometimes
>  		 * they will even be wakeup-enabled.
>  		 */
>  	}
>  
> -	musb_save_context(musb);
> -
>  	spin_unlock_irqrestore(&musb->lock, flags);
>  	return 0;
>  }
> 
> base-commit: 165768bb70265b5c38cf0b73fafd75be235f8b14