netdev - Re: [PATCH] r8169: Load MAC address from device tree if present

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Date:   Fri, 25 Jan 2019 20:23:29 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] r8169: Load MAC address from device tree if present

On 25.01.2019 20:07, Andrew Lunn wrote:
>> Andrew, for my understanding: What do you think is wrong with the 
>> alignment requirement? It was introduced because we do a 32 bit access
>> to the start address of the array and want to avoid an unaligned access.
> 
> Hi Heiner
> 
> Because you are doing pointer aliasing, the compiler will by default
> generate bad code, doing unaligned access. Adding the attribute works
> around this. But it is just a work around. Since this is very slow
> path code, i would just avoid the pointer aliasing, write a bit more C
> code as Thierry suggested, and the optimiser will probably figure out
> what is going on and produce reasonable code.
> 
> Also, in general, by avoiding pointer aliasing, you allow static code
> checkers to work better. They are more likely to discover buffer
> overruns, etc.
> 
>     Andrew
> 

Thanks, good to know.

The following doesn't hurt us here, but things like this have to be
considered too. According to chip spec:

"The ID registers 0-5 are only permitted to write by 4-byte access.
Read access can be byte, word, or double word access."

Heiner