Re: [RFC net-next 01/22] docs: ethtool: document that rx_buf_len must control payload lengths

[Joe Damato <jdamato@fastly.com>]

On Fri, Apr 25, 2025 at 03:50:34PM -0700, Jakub Kicinski wrote:
> On Wed, 23 Apr 2025 13:08:33 -0700 Mina Almasry wrote:
> > On Mon, Apr 21, 2025 at 3:28 PM Jakub Kicinski <kuba@kernel.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.

SGTM