linux-kernel - Re: [RFC 2/8] usb: dwc3: core: Register vendor hooks for dwc3-qcom

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Date:   Fri, 3 Nov 2023 15:14:49 +0000
From:   Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To:     Krishna Kurapati <quic_kriskura@...cinc.com>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>, quic_wcheng@...cinc.com
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        quic_ppratap@...cinc.com, quic_jackp@...cinc.com
Subject: Re: [RFC 2/8] usb: dwc3: core: Register vendor hooks for dwc3-qcom

On 17/10/2023 14:18, Krishna Kurapati wrote:
> Currently on QC targets, the conndone/disconnect events in device mode are
> generated by controller when software writes to QSCRATCH registers in qcom
> glue layer rather than the vbus line being routed to dwc3 core IP for it
> to recognize and generate these events.
> 
> We need to write '1' to  UTMI_OTG_VBUS_VALID bit of QSCRATCH_HS_PHY_CTRL
> register to generate a connection done event and "0" if we need to
> generate a disconnect event during cable removal or mode switch. Exactly
> what is done by "dwc3_qcom_vbus_override_enable" call in dwc3-qcom.
> 
> When the disconnect is not generated upon cable removal, the connected
> flag of dwc3 is left marked as "true" and it blocks runtime suspend.
> 
> The goal of these vendor hooks is to let the mode change and cable removal
> notifications from core reach the  glue layers so that glue can take
> necessary action.
> 
> Before flattening the device tree, glue driver is not sure when the core
> probe was completed as core probe can be deferred. In this case, glue is
> not sure when to register vendor hooks. So mandate enabling runtime only
> for flattened device node platforms so that glue can know when to register
> vendor hooks.
> 
> The following are the requirements aimed in this implementation:
> 
> 1. When enum in device mode, Glue/core must stay active.
> 
> 2. When cable is connected but UDC is not written yet, then glue/core
> must be suspended.
> 
> 3. Upon removing cable in device mode, the disconnect event must be
> generated and unblock runtime suspend for dwc3 core.
> 
> Signed-off-by: Krishna Kurapati <quic_kriskura@...cinc.com>

What happens to this code if you

static int count;

1. sleep in dwc3_probe for 10 milliseconds
2. return -EPROBE_DEFER
3. if count++ < 5 goto 1

i.e. if we simulate say waiting on a PHY driver to probe in dwc3_probe()

and what happens if we introduce a 100 millsecond sleep into 
dwc3_qcom_probe() - and run a fake disconnect event from 
dwc3_qcom_probe_core() directly ?

In other words if make it that dwc3_probe() completes and struct 
dwc3_glue_ops->notify_cable_disconnect() fires prior to 
dwc3_qcom_probe_core() completing ?

i.e. I don't immediately see how you've solved the probe() completion 
race condition here.

---
bod