netdev - Re: [PATCH net-next 2/2 v9] net: ethernet: Add a driver for Gemini gigabit ethernet

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Date:   Mon, 18 Dec 2017 14:54:03 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Tobias Waldvogel <tobias.waldvogel@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Paulius Zaleckas <paulius.zaleckas@...il.com>,
        netdev@...r.kernel.org,
        Hans Ulli Kroll <ulli.kroll@...glemail.com>,
        Janos Laube <janos.dev@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH net-next 2/2 v9] net: ethernet: Add a driver for Gemini
 gigabit ethernet

On Mon, Dec 18, 2017 at 03:48:17PM +0100, Michał Mirosław wrote:
> On Mon, Dec 18, 2017 at 02:57:37PM +0100, Linus Walleij wrote:
> > On Sat, Dec 16, 2017 at 8:39 PM, Linus Walleij <linus.walleij@...aro.org> wrote:
> > 
> > > The Gemini ethernet has been around for years as an out-of-tree
> > > patch used with the NAS boxen and routers built on StorLink
> > > SL3512 and SL3516, later Storm Semiconductor, later Cortina
> > > Systems. These ASICs are still being deployed and brand new
> > > off-the-shelf systems using it can easily be acquired.
> [...]
> > > ---
> > > Changes from v8:
> > > - Remove dependency guards in Kconfig to get a wider compile
> > >   coverage for the driver to detect broken APIs etc.
> > 
> > I guess we need to hold this off for a while, the code does
> > some weird stuff using the ARM-internal page DMA mapping
> > API.
> > 
> > I *think* what happens is that the driver allocates a global queue
> > used for RX and TX on both interfaces, then initializes that with
> > page pointers and gives that to the hardware to play with.
> > 
> > When an RX packet comes in, the RX routine needs to figure
> > out from the DMA (physical) address which remapped
> > page/address this random physical address pointer
> > corresponds to.
> > 
> > The Linux DMA API assumption is that the driver keeps track
> > of this mapping, not the hardware. So we need to figure out
> > a way to reverse-map this. Preferably quickly, and without
> > using any ARM-internal mapping APIs.
> 
> IIRC, the hardware copies descriptors from free queue (FREEQ)
> to RX queues. FREEQ is shared among the two ethernet ports.
> 
> This platform is CPU bound, so every additional lookup will
> hit performance here. In my version I had an #ifdef for
> COMPILE_TEST that replaced ARM-specific calls with stubs.
> Since the driver is not expected to work on other platforms,
> this seemed like the best workaround to make it compile
> on other arches.

Really.  No.  Stop going beneath the covers and using ARM private
implementation APIs in drivers.

Take that as a big NAK to that.

(I don't seem have the patch in question here to look at though.)

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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