linux-kernel - Re: [PATCH v20 2/5] usb: dwc3: core: Host wake up support from system suspend

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Date:   Fri, 1 Jul 2022 08:52:49 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Pavan Kondeti <quic_pkondeti@...cinc.com>
Cc:     Krishna Kurapati PSSNV <quic_kriskura@...cinc.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Felipe Balbi <balbi@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Doug Anderson <dianders@...omium.org>,
        Mathias Nyman <mathias.nyman@...el.com>,
        devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, quic_ppratap@...cinc.com,
        quic_vpulyala@...cinc.com
Subject: Re: [PATCH v20 2/5] usb: dwc3: core: Host wake up support from
 system suspend

On Fri, Jul 01, 2022 at 03:45:26PM +0530, Pavan Kondeti wrote:
> On Thu, Jun 30, 2022 at 06:10:50PM -0700, Matthias Kaehlcke wrote:
> > > > dwc3-qcom should wait for dwc3 core to call component_add() and then do
> > > > whatever needs to be done once the dwc3 core is registered in the
> > > > dwc3-qcom bind callback. Honestly this may all be a little overkill if
> > > > there's only two drivers here, dwc3-qcom and dwc3 core. It could
> > > > probably just be some callback from dwc3 core at the end of probe that
> > > > calls some function in dwc3-qcom.
> > > Since the issue we are facing is that the ssphy device links are not ready
> > > causing the dwc3 probe not being invoked, can we add an API as Pavan
> > > suggested
> > > to check if deferred_probe listfor dwc3 device is empty or not andbased on
> > > that we can choose to defer our qcomprobe ? In this case, we don't need to
> > > touch the dwc3 core driver and would be making changesonly in qcom glue
> > > driver.
> > 
> > As mentioned above, it shouldn't be necessary to add component support to
> > all the glue drivers. An API to check for deferred probing is an option,
> > however there is a possible race condition: When the dwc3-qcom driver checks
> > for a deferred probe the core could still be probing, in that situation the
> > glue would proceed before the core driver is ready. That could be avoided
> > with the component based approach.
> 
> The race can happen only if asynchronous probe is enabled, otherwise the
> child's probe happens synchronously in of_platform_populate() 

I was thinking about the case where the dwc3-qcom probe is initially deferred,
then the deferred probe starts shortly after (asynchronously) and now the
dwc3-qcom driver does its check. Probably it's not very likely to happen ...

> OTOH, would the below condition suffice for our needs here? if our device
> is not bounded to a driver, we check the state of initcalls and return
> either error or -EPROBE_DEFER
> 
> diff --git a/drivers/usb/dwc3/dwc3-qcom.c b/drivers/usb/dwc3/dwc3-qcom.c
> index 7b6eff5..519a503 100644
> --- a/drivers/usb/dwc3/dwc3-qcom.c
> +++ b/drivers/usb/dwc3/dwc3-qcom.c
> @@ -722,6 +722,9 @@ static int dwc3_qcom_of_register_core(struct platform_device *pdev)
>  		dev_err(dev, "failed to get dwc3 platform device\n");
>  	}
>  
> +	if (!qcom->dwc3->dev.driver)
> +		return driver_deferred_probe_check_state(&qcom->dwc3->dev);
> +
>  node_put:
>  	of_node_put(dwc3_np);

I like the simplicity of it, no need for new APIs.

The components based approach would be slightly safer, but in practice I
think this should be good enough.