linux-kernel - Re: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt driver

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Message-ID: <CAK8P3a0jnzse4sG58taO5+Yd5vCgh1uddqbtAuim_z9r15Q3BA@mail.gmail.com>
Date:   Thu, 31 Mar 2022 17:44:55 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Hitomi Hasegawa <hasegawa-hitomi@...itsu.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        SoC Team <soc@...nel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Doug Anderson <dianders@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 1/1] soc: fujitsu: Add A64FX diagnostic interrupt driver

On Thu, Mar 31, 2022 at 1:49 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > +
> > +static irqreturn_t a64fx_diag_handler(int irq, void *dev_id)
> > +{
> > +     handle_sysrq('c');
>
>
> Why is this calling this sysrq call?  From an interrupt?  Why?
>
> And you are hard-coding "c", are you sure?

This is an actual sysrq driver in the traditional sense, where you can send
a single interrupt to the machine from the outside over a side channel.

I suggested sysrq instead of just panic() to make it a bit more flexible.
Unfortunately there is no additional data, so it comes down to always
sending the same character.

It would be possible to make that character configurable with a module
parameter or something like that, but I'm not sure that is an improvement.
Maybe you have another idea for this.

> > +static void a64fx_diag_interrupt_clear(struct a64fx_diag_priv *priv)
> > +{
> > +     u32 mmsc;
> > +     const void __iomem *diag_status_reg_addr;
> > +
> > +     diag_status_reg_addr = priv->mmsc_reg_base + BMC_DIAG_INTERRUPT_STATUS_OFFSET;
> > +     mmsc = readl(diag_status_reg_addr);
> > +     if (mmsc & BMC_INTERRUPT_STATUS_MASK)
> > +             writel(BMC_INTERRUPT_STATUS_MASK, (void *)diag_status_reg_addr);
>
> No need to wait for the write to complete?
>
> You shouldn't have to cast diag_status_reg_addr, right?

I think the cast is needed because the declaration of
'diag_status_reg_addr' incorrectly
marks it as 'const'. However, this should still trigger a 'make C=1'
warning with sparse
because it is now missing the __iomem annotation.

The correct solution of course is to remove both the cast and the 'const'.

       Arnd