netdev - Re: [PATCH V6 00/20] net: ks8851: Unify KS8851 SPI and MLL drivers

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Message-Id: <20200516.190225.342589110126932388.davem@davemloft.net>
Date:   Sat, 16 May 2020 19:02:25 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     marex@...x.de
Cc:     netdev@...r.kernel.org, lukas@...ner.de, ynezz@...e.cz,
        yuehaibing@...wei.com
Subject: Re: [PATCH V6 00/20] net: ks8851: Unify KS8851 SPI and MLL drivers

From: Marek Vasut <marex@...x.de>
Date: Sun, 17 May 2020 02:33:34 +0200

> The KS8851SNL/SNLI and KS8851-16MLL/MLLI/MLLU are very much the same pieces
> of silicon, except the former has an SPI interface, while the later has a
> parallel bus interface. Thus far, Linux has two separate drivers for each
> and they are diverging considerably.
> 
> This series unifies them into a single driver with small SPI and parallel
> bus specific parts. The approach here is to first separate out the SPI
> specific parts into a separate file, then add parallel bus accessors in
> another separate file and then finally remove the old parallel bus driver.
> The reason for replacing the old parallel bus driver is because the SPI
> bus driver is much higher quality.

What strikes me in these changes is all of the new indirect jumps in
the fast paths of TX and RX packet processing.  It's just too much for
my eyes. :-)

Especially in the presence of Spectre mitigations, these costs are
quite non-trivial.

Seriously, I would recommend that instead of having these small
indirect helpers, just inline the differences into two instances of
the RX interrupt and the TX handler.