netdev - Re: [PATCH v2 net-next 12/15] net: ethernet: mtk_eth_soc: introduce MTK_NETSYS

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Date:   Wed, 18 May 2022 17:50:12 +0200
From:   Felix Fietkau <nbd@....name>
To:     Jakub Kicinski <kuba@...nel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, john@...ozen.org, sean.wang@...iatek.com,
        Mark-MC.Lee@...iatek.com, davem@...emloft.net, edumazet@...gle.com,
        pabeni@...hat.com, Sam.Shih@...iatek.com,
        linux-mediatek@...ts.infradead.org, devicetree@...r.kernel.org,
        robh@...nel.org, lorenzo.bianconi@...hat.com
Subject: Re: [PATCH v2 net-next 12/15] net: ethernet: mtk_eth_soc: introduce
 MTK_NETSYS_V2 support

On 18.05.22 17:47, Jakub Kicinski wrote:
> On Wed, 18 May 2022 11:53:12 +0200 Lorenzo Bianconi wrote:
>> > > +	WRITE_ONCE(desc->txd7, 0);
>> > > +	WRITE_ONCE(desc->txd8, 0);  
>> > 
>> > Why all the WRITE_ONCE()? Don't you just need a barrier between writing
>> > the descriptor and kicking the HW?   
>> 
>> I used this approach just to be aligned with current codebase:
>> https://github.com/torvalds/linux/blob/master/drivers/net/ethernet/mediatek/mtk_eth_soc.c#L1006
>> https://github.com/torvalds/linux/blob/master/drivers/net/ethernet/mediatek/mtk_eth_soc.c#L1031
>> 
>> but I guess we can even convert the code to use barrier instead. Agree?
> 
> Oh, I didn't realize. No preference on converting the old code
> but it looks like a cargo cult to me so in the new code let's
> not WRITE_ONCE() all descriptor writes unless there's a reason.
If I remember correctly, the existing places use WRITE_ONCE to prevent 
write tearing to uncached memory.

- Felix