netdev - Re: [PATCH] net: mdio: mdio-bcm-unimac: Delay before first poll

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Date: Wed, 13 Dec 2023 08:20:44 -0800
From: Florian Fainelli <florian.fainelli@...adcom.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>,
 Andrew Lunn <andrew@...n.ch>
Cc: Justin Chen <justin.chen@...adcom.com>, netdev@...r.kernel.org,
 Doug Berger <opendmb@...il.com>,
 Broadcom internal kernel review list
 <bcm-kernel-feedback-list@...adcom.com>,
 Heiner Kallweit <hkallweit1@...il.com>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: mdio: mdio-bcm-unimac: Delay before first poll

On 12/13/2023 7:01 AM, Russell King (Oracle) wrote:
> On Wed, Dec 13, 2023 at 11:57:52AM +0100, Andrew Lunn wrote:
>> On Tue, Dec 12, 2023 at 04:02:49PM -0800, Justin Chen wrote:
>>> With a clock interval of 400 nsec and a 64 bit transactions (32 bit
>>> preamble & 16 bit control & 16 bit data), it is reasonable to assume
>>> the mdio transaction will take 25.6 usec. Add a 30 usec delay before
>>> the first poll to reduce the chance of a 1000-2000 usec sleep.
>>
>> #define  MDIO_C45               0
>>
>> suggests the hardware can do C45? The timing works out different then.
>> Maybe add a comment by the udelay() that is assumes C22, to give a
>> clue to somebody who is adding C45 support the delay needs to be
>> re-evaluated.

Yes the hardware supports C45 though as Russell points out, the driver 
intentionally does not support it.

> 
> Note, however, that the driver only supports C22 operations (it only
> populates the read|write functions, not the c45 variants).
> 
> However, it doesn't explicitly set the MDIO_C22 bit in the configuration
> register, so what ends up being spat out on the bus would be dependent
> on the boot loader configuration.

The hardware is guaranteed to come up with the MDIO_C22 bit being set by 
default though it would not hurt setting it explicitly, that would be 
more future proof.

> 
> However, I'm wondering why unimac_mdio_poll() isn't written as
> (based on current code):
> 
> 	return read_poll_timeout(unimac_mdio_readl(priv, MDIO_CMD), val,
> 				 !(val & MDIO_START_BUSY), 2000,
> 				 2000000);
> 
> rather than open-coding the io polling.

The driver long predates the introduction of the iopoll.h header and its 
routines. That sounds like another change that could be made.
-- 
Florian

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