linux-kernel - Re: [PATCH net-next 2/2] net: stmmac: TBS support for platform driver

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Date: Fri, 26 Jan 2024 09:43:28 +0100
From: Esben Haabendal <esben@...nix.com>
To: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>
Cc: Rohan G Thomas <rohan.g.thomas@...el.com>,  "David S . Miller"
 <davem@...emloft.net>,  Alexandre Torgue <alexandre.torgue@...s.st.com>,
  "Jose Abreu" <joabreu@...opsys.com>,  Eric Dumazet <edumazet@...gle.com>,
  "Jakub Kicinski" <kuba@...nel.org>,  Paolo Abeni <pabeni@...hat.com>,
  Maxime Coquelin <mcoquelin.stm32@...il.com>,  Rob Herring
 <robh+dt@...nel.org>,  "Krzysztof Kozlowski"
 <krzysztof.kozlowski+dt@...aro.org>,  Conor Dooley <conor+dt@...nel.org>,
  Giuseppe Cavallaro <peppe.cavallaro@...com>,  "Serge Semin"
 <fancer.lancer@...il.com>,  Andrew Halaney <ahalaney@...hat.com>,
  <elder@...aro.org>,  <netdev@...r.kernel.org>,
  <linux-stm32@...md-mailman.stormreply.com>,
  <linux-arm-kernel@...ts.infradead.org>,  <devicetree@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>,  <quic_bhaviks@...cinc.com>,
  <kernel.upstream@...cinc.com>
Subject: Re: [PATCH net-next 2/2] net: stmmac: TBS support for platform driver

"Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com> writes:

> Qualcomm had similar discussions with respect to enabling of TBS for a
> particular queue. We had similar discussion on these terms yesterday
> with Redhat. Adding Andrew from Redhat here
>
> What we discovered as part of the discussions is listed below.
>
> 1. Today upstream stmmac code is designed in such a way that TBS flag
> is put as part of queue configurations(see below snippet) and as well
> know that stmmac queue configuration comes from the dtsi file.
>
> //ndo_open => stmmac_open
> int tbs_en = priv->plat->tx_queues_cfg[chan].tbs_en;(comes from tx_queues_cfg)
>
> /* Setup per-TXQ tbs flag before TX descriptor alloc */
> tx_q->tbs |= tbs_en ? STMMAC_TBS_AVAIL : 0;
>
> 2. There is a no way to do this dynamically from user space because we don't have any 
> API exposed which can do it from user space

Not now. But why not extend ethtool API to allow enabling TBS for
supported controllers?

> and also TBS rely on special descriptors aka enhanced desc this cannot
> be done run time and stmmac has to be aware of it before we do
> DMA/MAC/MTL start.

Isn't this somewhat similar to changing the RX/TX ring parameters, which
I believe also is quite difficult to do at run time, and ethtool
therefore requires the interface to be down in oroer to change them?

> To do this dynamically would only mean stopping DMA/MAC/MTL realloc
> resources for enhanced desc and the starting MAC/DMA/MTL. This means
> we are disrupting other traffic(By stopping MAC block).

Yes. But you would be disrupting traffic less than by requiring a
complete reboot of the target which is needed if the devicetree must be
changed.

> 3. I dont think there is a way we can enable this dynamically today. I
> would like upstream community to share your thoughts as well.

Hereby done. Could we investigate the possibility of using ethtool to
change TBS enable/disable "run-time"?

> 4. I agree with Rohan's patch here and want upstream community to
> accept it. This will allow use to configure the queues where TBS needs
> to be enabled as hardcoding in the code unless upstream has better way
> to this using userspace.
>
> Please let us know if you think otherwise. 

/Esben