linux-kernel - Re: [PATCH v2 6/9] usb: misc: qcom

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Message-ID: <f8169943-f986-4e9d-9d3b-a470e57f5e6a@oss.qualcomm.com>
Date: Wed, 4 Feb 2026 14:29:44 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Elson Serrao <elson.serrao@....qualcomm.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
 <conor+dt@...nel.org>,
        Souradeep Chowdhury <quic_schowdhu@...cinc.com>
Cc: linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/9] usb: misc: qcom_eud: add host mode coordination

On 1/27/26 12:38 AM, Elson Serrao wrote:
> EUD functions by presenting itself as a USB device to the host PC for
> debugging, making it incompatible in USB host mode configurations.
> Enabling EUD, when in host mode can also cause the USB controller to
> misbehave as the EUD hub can only have one upstream facing port.

Perhaps that's a silly idea, but would the device itself see
the debug hub, being able to essentially self-debug in a way?

[...]

> @@ -162,32 +165,66 @@ static ssize_t enable_store(struct device *dev,
>  		const char *buf, size_t count)
>  {
>  	struct eud_chip *chip = dev_get_drvdata(dev);
> +	struct eud_path *path;
>  	bool enable;
>  	int ret;
>  
>  	if (kstrtobool(buf, &enable))
>  		return -EINVAL;
>  
> +	mutex_lock(&chip->state_lock);

If you use guard(mutex)(&chip->state-lock), you can waive all these
conditional mutex_unlock additions

> +
>  	/* Skip operation if already in desired state */
> -	if (chip->enabled == enable)
> +	if (chip->enabled == enable) {
> +		mutex_unlock(&chip->state_lock);
>  		return count;
> +	}
> +
> +	/*
> +	 * Handle double-disable scenario: User is disabling EUD that was already
> +	 * disabled due to host mode. Since the hardware is already disabled, we
> +	 * only need to clear the host-disabled flag to prevent unwanted re-enabling
> +	 * when exiting host mode. This respects the user's explicit disable request.
> +	 */
> +	if (!enable && chip->eud_disabled_for_host) {
> +		chip->eud_disabled_for_host = false;
> +		chip->enabled = false;
> +		mutex_unlock(&chip->state_lock);
> +		return count;
> +	}
>  
>  	if (enable) {
> +		/*
> +		 * EUD functions by presenting itself as a USB device to the host PC for
> +		 * debugging, making it incompatible in USB host mode configuration.
> +		 * Prevent enabling EUD in this configuration to avoid hardware conflicts.
> +		 */
> +		path = chip->paths[chip->port_idx];
> +		if (path && path->curr_role == USB_ROLE_HOST) {
> +			dev_err(chip->dev, "EUD not usable in host mode configuration\n");

"in USB host mode"?

[...]

> +	/*
> +	 * EUD must be disabled when USB operates in host mode. EUD functions by
> +	 * presenting itself as a USB device to the host PC for debugging, making
> +	 * it incompatible in host mode configuration.
			   ^ with

Otherwise the logic looks good, I think this may be desired default
behavior (so that the user doesn't have to constantly keep re-enabling
EUD)

Konrad