linux-kernel - Re: [PATCH v2] watchdog: Add driver for Gunyah Watchdog

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Message-ID: <491de94c-e3c5-4f81-8e1a-82596413cede@quicinc.com>
Date: Mon, 6 Oct 2025 18:56:06 +0530
From: Pavan Kondeti <pavan.kondeti@....qualcomm.com>
To: Krzysztof Kozlowski <krzk@...nel.org>
Cc: Pavan Kondeti <pavan.kondeti@....qualcomm.com>,
        hrishabh.rajput@....qualcomm.com,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>, linux-arm-msm@...r.kernel.org,
        linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Neil Armstrong <neil.armstrong@...aro.org>
Subject: Re: [PATCH v2] watchdog: Add driver for Gunyah Watchdog

On Mon, Oct 06, 2025 at 10:03:59PM +0900, Krzysztof Kozlowski wrote:
> On 06/10/2025 19:03, Pavan Kondeti wrote:
> > On Mon, Oct 06, 2025 at 05:56:42PM +0900, Krzysztof Kozlowski wrote:
> >> On 06/10/2025 16:37, Hrishabh Rajput via B4 Relay wrote:
> >>> +
> >>> +static int __init gunyah_wdt_init(void)
> >>> +{
> >>> +	struct arm_smccc_res res;
> >>> +	struct watchdog_device *wdd;
> >>> +	struct device_node *np;
> >>> +	int ret;
> >>> +
> >>> +	np = of_find_compatible_node(NULL, NULL, "qcom,kpss-wdt");
> >>> +	if (np) {
> >>> +		of_node_put(np);
> >>> +		return -ENODEV;
> >>> +	}
> >>> +
> >>> +	np = of_find_compatible_node(NULL, NULL, "arm,sbsa-gwdt");
> >>> +	if (np) {
> >>> +		of_node_put(np);
> >>> +		return -ENODEV;
> >>> +	}
> >>> +
> >>> +	ret = gunyah_wdt_call(GUNYAH_WDT_STATUS, 0, 0, &res);
> >>> +	if (ret)
> >>> +		return -ENODEV;
> >>
> >> No, your hypervisor driver (which you have) should start the module via
> >> adding platform/aux/something devices. Now you are running this on every
> >> machine, which is clearly wrong...
> >>
> > 
> > This is a good point. Thanks for bringing it up. We don't have a
> > hypervisor glue driver (yet!) that can add an aux device. Based on v1
> > feedback, we would like to be a standalone module that can self discover
> > gunyah hypercall interface.
> > 
> > Currently this driver depends on ARCH_QCOM || COMPILE_TEST. So,
> > technically this can be built and loaded on all non-Qualcomm machines.
> 
> 
> Not technically, but practically. We do not make single-platform kernels
> anymore, it's not 2010. Entire arm64 is multiarch.

Thanks, I understand that we build single kernel image that works across
machines. However, I wonder do all modules built say from
arch/arm64/configs/defconfig gets loaded? Usually, the modules
corresponding to drivers for which devices are registered (modalias
based) gets loaded, correct? In this case, we don't have a device, so
there may be an explicit rule to load this module. I totally get your
point on why it would be preferred to make this module active only on
QCOM platform.

> 
> > 
> > We can make the STATUS SMCC before looking for the other watchdog
> > devices and fail early.
> > 
> > Our Gunyah glue driver [1] do make SMCC call to establish that we
> > are actually a guest under Gunyah. Since our intention here is to
> > support watchdog on as many as platform as possible, it is better not to
> > tie this with glue driver and make it a stand alone and self discovery
> > module.
> 
> 
> I think you should have only one driver pinging for Gunyah, so glue
> driver or this. Not both. If you add such SMC here, then how do you
> determine the platform in the glue driver? Via DT? Then DT supersedes this.

The SMCC that this module would be using is specific to Gunyah watchdog
interface. So there is no real dependency w/ Gunyah glue driver. I
understand your point that there should not be two drivers probing the
watchdog.

> 
> > 
> > If this is not an acceptable solution (Please let us know), we can find other 
> > ways to limit it to only work on Qualcomm machines. For ex: Socinfo
> > platform device is added from SMEM driver which make it only probed on 
> > Qualcomm machines. We can look into this. 
> 
> 
> To me socinfo feels even better. That way only, really only qcom devices
> will execute this SMC.
> 

Ok, we will look into this.

Thanks,
Pavan