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 21:55:36 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        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 3:54 PM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> 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.

Seems like that to me too. I will try to refactor and break it
apart a bit.

The way freeq works is undocumented, even in the official
datasheet for CS3516 (the memory area is just "reserved"),
so the code is the only documentation of it.

>> 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.

Don't worry, it won't happen. I am already thinking about better
approaches that stay with the public DMA-API.

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

I'll put you on CC in future postings.

Yours,
Linus Walleij