netdev - Re: [RFC PATCH net-next v4 1/2] macb: Add 1588 support in Cadence GEM.

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Date:   Tue, 3 Jan 2017 15:22:06 +0100
From:   Nicolas Ferre <nicolas.ferre@...el.com>
To:     Rafal Ozieblo <rafalo@...ence.com>,
        Harini Katakam <harinikatakamlinux@...il.com>,
        Richard Cochran <richardcochran@...il.com>
CC:     Andrei Pistirica <andrei.pistirica@...rochip.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "harini.katakam@...inx.com" <harini.katakam@...inx.com>,
        "punnaia@...inx.com" <punnaia@...inx.com>,
        "michals@...inx.com" <michals@...inx.com>,
        "anirudh@...inx.com" <anirudh@...inx.com>,
        "boris.brezillon@...e-electrons.com" 
        <boris.brezillon@...e-electrons.com>,
        "alexandre.belloni@...e-electrons.com" 
        <alexandre.belloni@...e-electrons.com>,
        "tbultel@...elsurmer.com" <tbultel@...elsurmer.com>
Subject: Re: [RFC PATCH net-next v4 1/2] macb: Add 1588 support in Cadence
 GEM.

Le 03/01/2017 à 11:47, Rafal Ozieblo a écrit :
>> From: Harini Katakam [mailto:harinikatakamlinux@...il.com] 
>> Sent: 3 stycznia 2017 06:06
>> Subject: Re: [RFC PATCH net-next v4 1/2] macb: Add 1588 support in Cadence GEM.
>>
>> Hi Richard,
>>
>> On Mon, Jan 2, 2017 at 9:43 PM, Richard Cochran <richardcochran@...il.com> wrote:
>>> On Mon, Jan 02, 2017 at 03:47:07PM +0100, Nicolas Ferre wrote:
>>>> Le 02/01/2017 à 12:31, Richard Cochran a écrit :
>>>>> This Cadence IP core is a complete disaster.
>>>>
>>>> Well, it evolved and propose several options to different SoC 
>>>> integrators. This is not something unusual...
>>>> I suspect as well that some other network adapters have the same 
>>>> weakness concerning PTP timestamp in single register as the early 
>>>> revisions of this IP.
>>>
>>> It appears that this core can neither latch the time on read or write, 
>>> or even latch time stamps.  I have worked with many different PTP HW 
>>> implementations, even early ones like on the ixp4xx, and it is no 
>>> exaggeration to say that this one is uniquely broken.
>>>
>>>> I suspect that Rafal tend to jump too quickly to the latest IP 
>>>> revisions and add more options to this series: let's not try to pour 
>>>> too much things into this code right now.
>>>
>>> Why can't you check the IP version in the driver?
>>
>> There is an IP revision register but it would be probably be
>> better to rely on "caps" from the compatibility strings - to cover SoC specific
>> implementations. Also, when this extended BD is added (with timestamp),
>> additional words will need to be added statically which will be
>> consistent with Andrei's CONFIG_ checks.
> We can distinguish IP cores with and without PTP support by reading
> Design Configuration Register. But to distinguish IP cores with
> timestamps in buffer descriptors and which support only event
> registers, we can only check IP version by reading the revision ID
> register and base on that.
> I agree with Harini, compatibility strings could be better. But we
> might end up with many different configuration in the future.

Compatibility strings and associated configurations are cheap. It's not
a problem to have many different configurations and clearer for this
particular "composite" feature.

> We could use only descriptor approach but there are many Atmel's
> cores on the market which support only event registers.

Yes and once in silicon, it's hard to modify ;-)

Regards,
-- 
Nicolas Ferre