linux-kernel - Re: [PATCH] soc: qcom: rpmh-rsc: Don't use ktime for timeout in write_tcs_reg

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Date:   Thu, 28 May 2020 15:44:48 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>
Cc:     Maulik Shah <mkshah@...eaurora.org>,
        Douglas Anderson <dianders@...omium.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] soc: qcom: rpmh-rsc: Don't use ktime for timeout in write_tcs_reg_sync()

Quoting Douglas Anderson (2020-05-28 07:48:34)
> The write_tcs_reg_sync() may be called after timekeeping is suspended
> so it's not OK to use ktime.  The readl_poll_timeout_atomic() macro
> implicitly uses ktime.  This was causing a warning at suspend time.
> 
> Change to just loop 1000000 times with a delay of 1 us between loops.
> This may give a timeout of more than 1 second but never less and is
> safe even if timekeeping is suspended.
> 
> NOTE: I don't have any actual evidence that we need to loop here.
> It's possibly that all we really need to do is just read the value
> back to ensure that the pipes are cleaned and the looping/comparing is
> totally not needed.  I never saw the loop being needed in my tests.
> However, the loop shouldn't hurt.
> 
> Fixes: 91160150aba0 ("soc: qcom: rpmh-rsc: Timeout after 1 second in write_tcs_reg_sync()")
> Reported-by: Maulik Shah <mkshah@...eaurora.org>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---

Reviewed-by: Stephen Boyd <sboyd@...nel.org>

Although I don't think ktime_get() inside of readl_poll_timeout_atomic()
is correct. The timekeeping base won't be able to update when a loop is
spinning in an irq disabled region. We need the tick interrupt to come
in and update the base. Spinning for a second with irqs disabled is also
insane for realtime so there's that problem too. Maybe we should try to
kick timekeeping forward from these loops manually. Anyway, not problems
with this patch so not important to fix immediately.