linux-kernel - Re: [RFC v2 3/5] clk: bcm2835: use firmware interface to update pllb

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Date:   Tue, 21 May 2019 14:18:53 +0200
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Petr Tesarik <ptesarik@...e.com>
Cc:     Oliver Neukum <oneukum@...e.com>, stefan.wahren@...e.com,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Eric Anholt <eric@...olt.net>, mbrugger@...e.de,
        viresh.kumar@...aro.org, rjw@...ysocki.net, sboyd@...nel.org,
        linux-rpi-kernel@...ts.infradead.org, ssuloev@...altech.com,
        linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        mturquette@...libre.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 3/5] clk: bcm2835: use firmware interface to update pllb

On Tue, 2019-05-21 at 14:14 +0200, Petr Tesarik wrote:
> On Tue, 21 May 2019 13:39:31 +0200
> Nicolas Saenz Julienne <nsaenzjulienne@...e.de> wrote:
> 
> > Hi Oliver, thanks for the review.
> > 
> > On Mon, 2019-05-20 at 14:43 +0200, Oliver Neukum wrote:
> > > On Mo, 2019-05-20 at 12:47 +0200, Nicolas Saenz Julienne wrote:  
> > > > + * For more information on the firmware interface check:
> > > > + * 
> > > > https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface
> > > > + */
> > > > +struct bcm2835_firmware_prop {
> > > > +       u32 id;
> > > > +       u32 val;
> > > > +       u32 disable_turbo;
> > > > +} __packed;  
> > > 
> > > Hi,
> > > 
> > > technically we are not in arch and those fields have a defined
> > > endianness.
> > >   
> > 
> > Well I set it as packed since it's 'sent' through a memory mapped firmware
> > interface. Hence the need for the structure format to be fixed. So I guessed
> > we're safer with it, as I'm not 100% sure what the different compilers are
> > going to do with it (although it's very likely it'll stay the same). BTW
> > this
> > will be built both for arm & arm64.
> 
> I believe that's not the point Oliver was trying to make. You should
> use __le32 instead of u32.
> 
> That's because u32 means "host byte order" and this code is not located
> under arch/, so host endianness is unknown, but the mailbox interface
> requires little-endian.
> 
> It's nit-picking, and that's why Oliver writes 'technically'; there is
> probably no way this firmware interface could be used on a big-endian
> CPU...

Understood, thanks for the clarification.

Regards,
Nicolas


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