Re: [PATCH] xhci: tegra: Delay for disabling LFPS detector

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Dec 19, 2020 at 12:42:34AM +0800, JC Kuo wrote:
> Occasionally, we are seeing some SuperSpeed devices resumes right after
> being directed to U3. This commits add 500us delay to ensure LFPS
> detector is disabled before sending ACK to firmware.
> 
> [   16.099363] tegra-xusb 70090000.usb: entering ELPG
> [   16.104343] tegra-xusb 70090000.usb: 2-1 isn't suspended: 0x0c001203
> [   16.114576] tegra-xusb 70090000.usb: not all ports suspended: -16
> [   16.120789] tegra-xusb 70090000.usb: entering ELPG failed
> 
> Signed-off-by: JC Kuo <jckuo@nvidia.com>
> ---
>  drivers/usb/host/xhci-tegra.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/usb/host/xhci-tegra.c b/drivers/usb/host/xhci-tegra.c
> index 934be1686352..20cdc11f7dc6 100644
> --- a/drivers/usb/host/xhci-tegra.c
> +++ b/drivers/usb/host/xhci-tegra.c
> @@ -623,6 +623,12 @@ static void tegra_xusb_mbox_handle(struct tegra_xusb *tegra,
>  								     enable);
>  			if (err < 0)
>  				break;
> +
> +			/*
> +			 * wait 500us for LFPS detector to be disabled before sending ACK
> +			 */
> +			if (!enable)
> +				usleep_range(500, 1000);

Where does the magic 500us come from?  How can we "know" this is long
enough?

thanks,

greg k-h