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

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Date:   Tue, 3 Jan 2017 10:47:56 +0000
From:   Rafal Ozieblo <rafalo@...ence.com>
To:     Harini Katakam <harinikatakamlinux@...il.com>,
        Richard Cochran <richardcochran@...il.com>
CC:     Nicolas Ferre <nicolas.ferre@...el.com>,
        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.

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