Re: [PATCH v3] net: ethernet: mediatek: ppe: add support for flow accounting

From: Felix Fietkau <nbd@nbd.name>
To: Andrew Lunn <andrew@lunn.ch>, Daniel Golle <daniel@makrotopia.org>
Cc: John Crispin <john@phrozen.org>,
	Sean Wang <sean.wang@mediatek.com>,
	Mark Lee <Mark-MC.Lee@mediatek.com>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	netdev@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] net: ethernet: mediatek: ppe: add support for flow accounting
Date: Wed, 2 Nov 2022 06:49:15 +0100	[thread overview]
Message-ID: <770dd173-41b8-80f7-7c78-40319734cfbf@nbd.name> (raw)
In-Reply-To: <Y2HHOSaYQGAhWL/E@lunn.ch>

On 02.11.22 02:26, Andrew Lunn wrote:
> On Wed, Nov 02, 2022 at 01:03:08AM +0000, Daniel Golle wrote:
>> Hi Andrew,
>> 
>> On Wed, Nov 02, 2022 at 01:51:21AM +0100, Andrew Lunn wrote:
>> > On Wed, Nov 02, 2022 at 12:42:40AM +0000, Daniel Golle wrote:
>> > > The PPE units found in MT7622 and newer support packet and byte
>> > > accounting of hw-offloaded flows. Add support for reading those
>> > > counters as found in MediaTek's SDK[1].
>> > > 
>> > > [1]: https://git01.mediatek.com/plugins/gitiles/openwrt/feeds/mtk-openwrt-feeds/+/bc6a6a375c800dc2b80e1a325a2c732d1737df92
>> > > Signed-off-by: Daniel Golle <daniel@makrotopia.org>
>> > 
>> > Sorry, but NACK.
>> > 
>> > You have not explained why doing this correctly via ethtool -S cannot
>> > be done. debugfs is a vendor crap way of doing this.
>> 
>> The debugfs interface is pre-existing and **in addition** to the
>> standard Linux interfaces which are also provided. It is true that
>> the debugfs interface in this case doesn't provide much additional
>> value apart from having the counter listed next to the hardware-
>> specific hashtable keys. As the debugfs interface for now aims to
>> be as complete as possible, naturally there is some redundance of
>> things which can also be accessed using other (standard) interfaces.
> 
> debugfs is by definition unstable. It is not ABI. Anything using it is
> expected to break in the near future when it changes its layout. It is
> also totally option, you cannot expect it to be mounted.
> 
> I hope you don't have any user space code using it.
> 
> Maybe i should submit a patch which just for the fun of it rearranged
> the order in debugfs and change the file name?

I believe that OpenWrt is still the main user of the PPE offloading 
code, since most vendors of devices with these SoC still use different 
out-of-tree implementations.
OpenWrt does not ship or contain any user space code that relies on 
these debugfs files.

Whenever I'm debugging PPE related issues, I rely heavily on the debugfs 
API, so keeping it as complete as possible is important to me as well.

Aside from that, exposing per-flow statistics (which is what this patch 
does) via ethtool -S API makes no sense to me at all. I'm not aware of 
any other offload capable driver that does this.

- Felix

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