linux-kernel - Re: [PATCH] firmware: arm_scmi: Use {get,put}_unaligned

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Message-ID: <20190807170808.GD27278@e107155-lin>
Date:   Wed, 7 Aug 2019 18:08:09 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>
Subject: Re: [PATCH] firmware: arm_scmi: Use {get,put}_unaligned_le32
 accessors

On Wed, Aug 07, 2019 at 03:18:59PM +0000, David Laight wrote:
> From: Sudeep Holla
> > Sent: 07 August 2019 14:01
> >
> > Instead of type-casting the {tx,rx}.buf all over the place while
> > accessing them to read/write __le32 from/to the firmware, let's use
> > the nice existing {get,put}_unaligned_le32 accessors to hide all the
> > type cast ugliness.
>
> Why the 'unaligned' accessors?
>

Since the firmware run in LE, we do byte-swapping anyways.

> > -	*(__le32 *)t->tx.buf = cpu_to_le32(id);
> > +	put_unaligned_le32(id, t->tx.buf);
>

If you look at the generic definition for put_unaligned_le32, it's
exactly the same as what I am replacing with the call. So nothing
changes IIUC. In fact, I see that all the helper in unaligned/access_ok.h
just do the byte-swapping.

> These will be expensive if the cpu doesn't support them.

The SCMI is currently used only on ARM platforms which have
HAVE_EFFICIENT_UNALIGNED_ACCESS defined.

--
Regards,
Sudeep