linux-kernel - Re: [PATCH net-next v3 2/7] net: phy: microchip_t1s: update new initial settings for LAN865X Rev.B0

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Message-ID: <61c1ccf2-1eca-43be-84db-e979de0ae0f5@microchip.com>
Date: Tue, 8 Oct 2024 05:47:10 +0000
From: <Parthiban.Veerasooran@...rochip.com>
To: <kuba@...nel.org>
CC: <andrew@...n.ch>, <hkallweit1@...il.com>, <linux@...linux.org.uk>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <pabeni@...hat.com>,
	<ramon.nordin.rodriguez@...roamp.se>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <UNGLinuxDriver@...rochip.com>,
	<Thorsten.Kummermehr@...rochip.com>
Subject: Re: [PATCH net-next v3 2/7] net: phy: microchip_t1s: update new
 initial settings for LAN865X Rev.B0

Hi Jakub,

On 07/10/24 9:30 pm, Jakub Kicinski wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Mon, 7 Oct 2024 07:51:36 +0000 Parthiban.Veerasooran@...rochip.com
> wrote:
>> On 05/10/24 12:20 am, Jakub Kicinski wrote:
>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>
>>> On Tue, 1 Oct 2024 18:07:29 +0530 Parthiban Veerasooran wrote:
>>>> +     cfg_results[0] = FIELD_PREP(GENMASK(15, 10), (9 + offsets[0]) & 0x3F) |
>>>> +                      FIELD_PREP(GENMASK(15, 4), (14 + offsets[0]) & 0x3F) |
>>>> +                      0x03;
>>>> +     cfg_results[1] = FIELD_PREP(GENMASK(15, 10), (40 + offsets[1]) & 0x3F);
>>>
>>> It's really strange to OR together FIELD_PREP()s with overlapping
>>> fields. What's going on here? 15:10 and 15:4 ranges overlap, then
>>> there is 0x3 hardcoded, with no fields size definition.
>> This calculation has been implemented based on the logic provided in the
>> configuration application note (AN1760) released with the product.
>> Please refer the link [1] below for more info.
>>
>> As mentioned in the AN1760 document, "it provides guidance on how to
>> configure the LAN8650/1 internal PHY for optimal performance in
>> 10BASE-T1S networks." Unfortunately we don't have any other information
>> on those each and every parameters and constants used for the
>> calculation. They are all derived by design team to bring up the device
>> to the nominal state.
>>
>> It is also mentioned as, "The following parameters must be calculated
>> from the device configuration parameters mentioned above to use for the
>> configuration of the registers."
>>
>> uint16 cfgparam1 = (uint16) (((9 + offset1) & 0x3F) << 10) | (uint16)
>> (((14 + offset1) & 0x3F) << 4) | 0x03
>> uint16 cfgparam2 = (uint16) (((40 + offset2) & 0x3F) << 10)
>>
>> This is the reason why the above logic has been implemented.
> 
> In this case the code should simply be:
> 
>       cfg_results[0] = FIELD_PREP(GENMASK(15, 10), 9 + offsets[0]) |
>                        FIELD_PREP(GENMASK(9, 4), 14 + offsets[0]) |
> 
> the fields are clearly 6b each. FILED_PREP() already masks.
Ah ok, thanks for the input. I will take care in other places as well.
> 
>>> Could you clarify and preferably name as many of the constants
>>> as possible?
>> I would like to do that but as I mentioned above there is no info on
>> those constants in the application note.
>>>
>>> Also why are you masking the result of the sum with 0x3f?
>>> Can the result not fit? Is that safe or should we error out?
>> Hope the above info clarifies this as well.
>>>
>>>> +             ret &= GENMASK(4, 0);
>>> ?               if (ret & BIT(4))
>>>
>>> GENMASK() is nice but naming the fields would be even nicer..
>>> What's 3:0, what's 4:4 ?
>> As per the information provided in the application note, the offset
>> value expected range is from -5 to 15. Offsets are stored as signed
>> 5-bit values in the addresses 0x04 and 0x08. So 0x1F is used to mask the
>> 5-bit value and if the 4th bit is set then the value from 27 to 31 will
>> be considered as -ve value from -5 to -1.
>>
>> I think adding the above comment in the above code snippet will clarify
>> the need. What do you think?
> 
> Oh yes, a comment, e.g. /* 5-bit signed value, sign extend */
> would help a lot, thanks!
Sure, I will add this comment in the code snippet. Thanks.

Best regards,
Parthiban V