netdev - Re: [RFC net-next 01/22] docs: ethtool: document that rx_buf

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Message-ID: <20250425155034.096b7d55@kernel.org>
Date: Fri, 25 Apr 2025 15:50:34 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Mina Almasry <almasrymina@...gle.com>, Joe Damato <jdamato@...tly.com>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
 pabeni@...hat.com, andrew+netdev@...n.ch, horms@...nel.org,
 donald.hunter@...il.com, sdf@...ichev.me, dw@...idwei.uk,
 asml.silence@...il.com, ap420073@...il.com, dtatulea@...dia.com,
 michael.chan@...adcom.com
Subject: Re: [RFC net-next 01/22] docs: ethtool: document that rx_buf_len
 must control payload lengths

On Wed, 23 Apr 2025 13:08:33 -0700 Mina Almasry wrote:
> On Mon, Apr 21, 2025 at 3:28 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > @@ -971,6 +970,11 @@ completion queue size can be adjusted in the driver if CQE size is modified.
> >  header / data split feature. If a received packet size is larger than this
> >  threshold value, header and data will be split.
> >
> > +``ETHTOOL_A_RINGS_RX_BUF_LEN`` controls the size of the buffer chunks driver
> > +uses to receive packets. If the device uses different memory polls for headers
> > +and payload this setting may control the size of the header buffers but must
> > +control the size of the payload buffers.
> 
> FWIW I don't like the ambiguity that the setting may or may not apply
> to header buffers. AFAIU header buffers are supposed to be in the
> order of tens/hundreds of bytes while the payload buffers are 1-2
> orders of magnitude larger. Why would a driver even want this setting
> to apply for both? I would prefer this setting to apply to only
> payload buffers.

Okay, I have no strong reason to leave the ambiguity.

Converging the thread with Joe:

>> Document the semantics of the rx_buf_len ethtool ring param.
>> Clarify its meaning in case of HDS, where driver may have
>> two separate buffer pools.  
>
> FWIW the docs added below don't explicitly mention HDS, but I
> suppose that is implied from the multiple memory pools? Not sure if
> it's worth elucidating that.

Maybe not sufficiently. Some NICs just have buffer pools for different
sized packets, but than the buffer size should be implied by the size
range?


How about:

 ``ETHTOOL_A_RINGS_RX_BUF_LEN`` controls the size of the buffers driver
 uses to receive packets. If the device uses different buffer pools for
 headers and payload (due to HDS, HW-GRO etc.) this setting must
 control the size of the payload buffers.