linux-kernel - Re: [PATCH 1/6] phy: Add max_bitrate attribute & phy_get_max

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Date:   Mon, 5 Nov 2018 11:57:41 +0530
From:   Faiz Abbas <faiz_abbas@...com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>,
        <linux-kernel@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <netdev@...r.kernel.org>, <linux-can@...r.kernel.org>
CC:     <wg@...ndegger.com>, <robh+dt@...nel.org>, <mark.rutland@....com>,
        <kishon@...com>
Subject: Re: [PATCH 1/6] phy: Add max_bitrate attribute &
 phy_get_max_bitrate()

Hi Marc,

On Saturday 03 November 2018 03:06 PM, Marc Kleine-Budde wrote:
> On 11/02/2018 08:26 PM, Faiz Abbas wrote:
>> In some subsystems (eg. CAN) the physical layer capabilities are
>> the limiting factor in the datarate of the device. Typically, the
>> physical layer transceiver does not provide a way to discover this
>> limitation at runtime. Thus this information needs to be represented as
>> a phy attribute which is read from the device tree.
>>
>> Therefore, add an optional max_bitrate attribute to the generic phy
>> sybsystem. Also add the complementary API which enables the consumer
>> to get max_bitrate.
>>
>> Signed-off-by: Faiz Abbas <faiz_abbas@...com>
> 
> NACK - We already have such a functionality in the CAN subsystem.
> Please have a look at the patches:
> 
> e759c626d826 can: m_can: Support higher speed CAN-FD bitrates
> b54f9eea7667 dt-bindings: can: m_can: Document new can transceiver binding
> 2290aefa2e90 can: dev: Add support for limiting configured bitrate
> 54a7fbcc17bc dt-bindings: can: can-transceiver: Document new binding
> 

I remove the transceiver child node binding documentation in patch 5/6.

The existing implementation is pretty limiting as it just has a child
node with no associated device. What if a transceiver requires its own
configurations before it can start sending/receiving messages (for
example, my usecase requires it to pull the standby line low)?

I think that can be solved by implementing the transceiver as a phy and
exposing a generic get_max_bitrate API. That way, the transceiver device
can do all its startup configuration in the phy probe function.

In any case, do suggest if you have a better idea on how to implement
pull gpio low requirement.

Thanks,
Faiz