linux-kernel - Re: [PATCH] Add Ingenic JZ4780 hardware RNG driver

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Date:   Fri, 19 Aug 2016 18:39:00 +0530
From:   PrasannaKumar Muralidharan <prasannatsmkumar@...il.com>
To:     noloader@...il.com
Cc:     linux-crypto@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org
Subject: Re: [PATCH] Add Ingenic JZ4780 hardware RNG driver

> __ARM_FEATURE_UNALIGNED (cf.,
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0774f/chr1383660321827.html)
> . MIPSEL does not define such a macro.
>
>     # MIPS ci20 creator with GCC 4.6
>     $ gcc -march=native -dM -E - </dev/null | grep -i align
>     #define __BIGGEST_ALIGNMENT__ 8
>
> If the MIPS CPU does not tolerate unaligned data access, then the
> following could SIGBUS:
>
>> +       u32 *data = buf;
>> +       *data = jz4780_rng_readl(jz4780_rng, REG_RNG_DATA);
>
> If GCC emits code that uses the MIPS unaligned load and store
> instructions, then there's probably going to be a performance penalty.
>
> Regardless of what the CPU tolerates, I believe unaligned data access
> is undefined behavior in C/C++. I believe you should memcpy the value
> into the buffer.

I am not sure whether this is required. 'buf' is part of a structure
so I guess it is properly aligned by padding. Not sure though, will
look about this.