linux-kernel - Re: [PATCH v6 2/2] clocksource: add J-Core timer/clocksource driver

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Message-ID: <1815946.6MSqSqog7A@wuerfel>
Date:   Thu, 25 Aug 2016 12:23:48 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Rich Felker <dalias@...c.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <Marc.Zyngier@....com>
Subject: Re: [PATCH v6 2/2] clocksource: add J-Core timer/clocksource driver

On Wednesday, August 24, 2016 11:57:08 PM CEST Arnd Bergmann wrote:
> > I'm not familiar with those classifications, but from what I can tell,
> > BE32 describes it correctly. I'll see if I can get someone to verify
> > this. Is there a reason it's not widely used anymore? Perhaps
> > something related to supporting misaligned word-sized loads/stores?
> 
> The main problem I see is that you can't easily use MMIO registers that
> are not 32-bit wide -- you end up having to flip not just the register
> contents but also the lower bits of the address in order to reach
> the right contents of the register.
> 

Actually there is a much more serious problem with BE32/LE32 mode:
doing unaligned access to RAM is a huge pain at the HW level,
and also when emulating it in the kernel on CPUs that require
aligned access. In ARM THUMB2 mode, you can even have unaligned
32-bit instructions.

	Arnd