netdev - Re: [PATCH v2 1/6] net: wan: Add support for QMC HDLC

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Message-ID: <20240205173550.2a6b8f20@bootlin.com>
Date: Mon, 5 Feb 2024 17:35:50 +0100
From: Herve Codina <herve.codina@...tlin.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: Vadim Fedorenko <vadim.fedorenko@...ux.dev>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
 <kuba@...nel.org>, linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, Andrew Lunn <andrew@...n.ch>, Mark Brown
 <broonie@...nel.org>, Christophe Leroy <christophe.leroy@...roup.eu>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v2 1/6] net: wan: Add support for QMC HDLC

On Mon, 05 Feb 2024 16:49:33 +0100
Paolo Abeni <pabeni@...hat.com> wrote:

[...
> > > 
> > > In general is quite bad that the existing infra does not allow
> > > leveraging NAPI. Have you considered expanding the QMC to accomodate
> > > such user?  
> > 
> > I cannot mask/unmask the 'end of transfer' interrupt.
> > Indeed, other streams use this interrupt among them audio streams and so
> > masking the interrupt for HDLC data will also mask the interrupt for audio
> > data.  
> 
> Uhm... I fear the above makes the available options list empty :(
> 
> > At the HDLC driver level, the best I can to is to store a queue of complete
> > HDLC skbs (queue filled on interrupts) and send them to the network stack
> > when the napi poll() is called.
> > 
> > I am not sure that this kind of queue (additional level between always
> > enabled interrupts and the network stack) makes sense.
> > 
> > Do you have any opinion about this additional queue management for NAPI
> > support?  
> 
> With such idea in place, what HDLC-level data will be accessed by the
> napi context? The RX interrupts will remain unmasked after the
> interrupt and before the napi poll right? That would be
> problematic/could cause drop if the ingress pkt/interrupt rate will be
> higher that what the napi could process - and that in turn could bring
> back old bad livelock times :(

Indeed.
So the best thing to do is to keep this driver without NAPI support.

Best regards,
Hervé