linux-kernel - Re: [PATCH v3 1/3] x86: baytrail/cherrytrail: Rework and move P-Unit PMIC bus semaphore code

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Date:   Fri, 12 Oct 2018 18:08:21 +0100
From:   Alan Cox <gnomes@...rguk.ukuu.org.uk>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, linux-i2c@...r.kernel.org,
        linux-acpi@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] x86: baytrail/cherrytrail: Rework and move
 P-Unit PMIC bus semaphore code

> > It should be.  
> 
> You mean that the problem should be purely academic, IOW that registers touched
> by the P-Unit are never touched through ACPI Opregions / power-resources?

As far as I am aware. Holding the lock over both is definitely better
regardless

> >> 2) To safely access the shared I2C bus, we need to do 3 things:
> >> a) Notify the GPU driver that we are starting a window in which it may not
> >> access the P-Unit, since the P-Unit seems to ignore the semaphore for
> >> explicit power-level requests made by the GPU driver  
> > 
> > That's not what happens. It's more a problem of
> > 
> > We take the SEM
> > The GPU driver pokes the GPU
> > The GPU decides it wants to change the power situation
> > The GPU asks
> > It blocks on the SEM
> > 
> > and the system deadlocks.  
> 
> That may be, but why does it deadlock?

As I understand it because the CPU is stuck waiting for the GPU which is
waiting for the SEM which the CPU is holding. This isn't purely software
remember.

> I can understand that you are reluctant to change this code, but this
> commit is not changing the logic, it mostly just moves the code around
> and I do believe that overall doing this is worthwhile.

Fair enough

Alan