netdev - Re: [PATCH/RFC net-next 3/5] ravb: do not write 1 to reserved bits

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Date:   Sat, 21 Apr 2018 23:44:47 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     David Miller <davem@...emloft.net>, horms+renesas@...ge.net.au
Cc:     magnus.damm@...il.com, netdev@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        wsa+renesas@...g-engineering.com, kazuya.mizuguchi.ks@...esas.com
Subject: Re: [PATCH/RFC net-next 3/5] ravb: do not write 1 to reserved bits

Hello!

On 04/17/2018 05:15 PM, David Miller wrote:

>> From: Kazuya Mizuguchi <kazuya.mizuguchi.ks@...esas.com>
>>
>> This patch corrects writing 1 to reserved bits.
>> The write value should be 0.
>>
>> Signed-off-by: Kazuya Mizuguchi <kazuya.mizuguchi.ks@...esas.com>
>> Signed-off-by: Simon Horman <horms+renesas@...ge.net.au>
> 
> How are we ending up in situations where the driver is trying to write
> non-zero values to those fields in the first place?

   The brain damaged AVB core design is to blame here. You have to write 0 to
clear the set bits and, at the same time, you can't write 1 to the reserved
bits... :-/

> The places creating those values should be making sure that the
> reserved bits are never set.

   That's basically what this patch is doing.

> If you mask out the reserved bits in the register writing locations,
> this just hides bugs.

   There are no *other* locations in some cases... 
   And I don't think that forcing the reserved bits to 1 after a register is
read would look better. :-(

MBR, Sergei