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

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Message-ID: <0fd2cff2-176e-419d-9e11-a98150266fee@oss.qualcomm.com>
Date: Mon, 9 Feb 2026 09:37:22 -0800
From: Elson Serrao <elson.serrao@....qualcomm.com>
To: Konrad Dybcio <konrad.dybcio@....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 2/4/2026 5:29 AM, Konrad Dybcio wrote:
> 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?
> 

This isn’t supported by the current hardware topology.

When EUD is enabled, it enumerates as a USB device to an external host via its
upstream-facing port, while the SoC USB controller sits behind the hub’s
downstream-facing port. As a result, the controller cannot enumerate or
interact with the EUD device itself, and host mode is mutually exclusive with
EUD debug on this path.

Thanks
Elson

> [...]
> 
>> @@ -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