linux-kernel - Re: [PATCH] media: venus: Synchronize probe() between venus

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Message-ID: <00b817a4-f1ac-6a94-5f1e-836d8d313406@linaro.org>
Date:   Mon, 25 Oct 2021 17:03:52 +0100
From:   Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To:     Tadeusz Struk <tadeusz.struk@...aro.org>,
        Stanimir Varbanov <stanimir.varbanov@...aro.org>,
        Andy Gross <agross@...nel.org>
Cc:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Amit Pundir <amit.pundir@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        linux-media@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: venus: Synchronize probe() between venus_core and
 enc/dec

On 25/10/2021 16:30, Tadeusz Struk wrote:
> No, we want a hard stop here. At this point the venus_core probe()
> has finished and it failed. Returning -EPROBE_DEFER here will just
> cause it to loop infinitely.

I don't think there's any guarantee at all, that core probe() has 
completed at that point.

of_platform_populate() doesn't guarantee ordering of the probe() 
completing before or after the probe() of the platform drivers that are 
associated with the devices in of_platform_populate().

When you think it about it can't do that and you wouldn't want it to do 
that since a device might have a legitimate reason to EPROBE_DEFER

As an example core could call of_platform_populate() and then as a 
ridiculous example go to sleep for five seconds - in which case it is 
perfectly possible the encoder and decoder probe() functions will bug 
out illegitimately waiting because of core->state != CORE_INIT

This is a problem we have and still haven't solved in 
drivers/usb/dwc3/dwc3-qcom.c::probe() and child devices 
drivers/usb/dwc3/dwc3-qcom.c

Here:
https://patches.linaro.org/cover/470387/

There's no serialisation guarantee between parent and child on 
of_platform_populate() - at least none I'm aware of.

---
bod