Message-ID: <d8aaea9b-dc3a-66ba-6bd3-e400be204013@ti.com>
Date:   Thu, 22 Dec 2022 23:25:45 +0530
From:   Apurva Nandan <a-nandan@...com>
To:     Nishanth Menon <nm@...com>, Andrew Davis <afd@...com>
CC:     Vignesh Raghavendra <vigneshr@...com>,
        Tero Kristo <kristo@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-gpio@...r.kernel.org>, Hari Nagalla <hnagalla@...com>
Subject: Re: [PATCH v3 4/4] arm64: dts: ti: Add support for J784S4 EVM board


On 19/11/22 03:45, Nishanth Menon wrote:
> On 15:08-20221118, Andrew Davis wrote:
>> On 11/18/22 1:27 PM, Nishanth Menon wrote:
>>> On 12:15-20221118, Andrew Davis wrote:
>>>> I don't see either of those addressed in that thread, only that
>>>> the aliases should go in the .dts files and be trimmed, nothing
>>> Key is trimmed to what the system and ecosystem needs.
>>>
>>>> stops us from:
>>>>
>>>> chosen {
>>>> 	stdout-path = "serial10:115200n8";
>>>> };
>>>>
>>>> aliases {
>>>> 	serial10 = &main_uart8;
>>>> };
>>> Do we need 10 serial aliases? There are'nt 10 serial ports exposed in
>>> j782s2. ok - lets say we do this, then: [1] is needed to boot? but why
>>> do we need to do that for all armv8 platforms when aliases allows us
>> Why do we need SERIAL_8250_NR_UARTS at all, might be a better question.
>> These should be dynamically allocated if the number goes over the
>> default count imposed by the TTY framework. Maybe folks are still a
>> bit too afraid to touch the TTY subsystem core, I don't blame them..
>>
>>> to trim it to just the 3 or 4 serial ports the platform really needs
>>> That + being able to use the convention that serial2 is always linux
>>> console, is'nt that a good thing? Hence recommending to just expose the
>>> serialports as aliases to exactly what we need while keeping serial2 as
>>> the linux console (which in this case happens to be main_uart8 - example
>>> as j721s2 does).
>>>
>> "serial2 as the linux console" is *not* a convention, we just don't want to
>> fix up our bootloader/userspace to actually reason about what serial ports to
>> put logins on. Why not make ttyS10 the default, or ttyS666, it doesn't solve
>> your multi-distro issue either way since they usually only start a login on
>> ttyS0, console=, and/or the first virtual tty. Never on ttyS2. So you are
>> hacking up DT for a solution that doesn't do what you want in the end.
> ttyS2 is an accidental convention not a "by design" or definition
> convention. I suspect we ended up here from old OMAP days - all
> platforms in k3 ended up with ttyS2. In hindsight, if I had to do it
> by design, I would probably have picked ttyS0, well, we did'nt.
>
> $ git grep stdout-path arch/arm/boot/dts|grep serial|cut -d '=' -f 2|cut -d ':' -f1|grep -v '&'|sort|uniq -c
>      379  "serial0
>       21  "serial1
>       33  "serial2
>       13  "serial3
>        1  "/slaves@...00000/serial@0
>
> $ git grep stdout-path arch/arm64/boot/dts|grep serial|cut -d '=' -f 2|cut -d ':' -f1|grep -v '&'|sort|uniq -c
>      245  "serial0
>       17  "serial0";
>        7  "serial1
>       49  "serial2
>        3  "serial3
>        2  "serial4
>        2  "serial5
>        2  "serial6
>
> $ git grep stdout-path arch/arm64/boot/dts/ti|grep serial|cut -d '=' -f 2|cut -d ':' -f1|grep -v '&'|sort|uniq -c
>        8  "serial2
>        1  "serial3
>
> I don't buy the argument here for 1-1 mapping of aliased serial
> instances to instances - why should main_uart8 be ttyS10, why not
> ttyS8 (it is equally valid - why was it called uart8?).. That mapping
> is just a convention we are choosing to create.
>
> The iot2050 ecosystem picked ttyS3 as the linux console for reasons of
> that ecosystem. K3 so far has selected ttyS2 as the convention for
> console - no matter where the serial instances have been located.
>
> I want to maintain consistency of existing TI platforms here without
> needing to shove a dozen things on existing users (yes j78-evm is a new
> board, but it is within the existing k3 s/w ecosystem and yes, getty,
> systemd etc are smarter today than once upon a time)
>
> So, given TI K3 history does'nt follow rest of the non-TI instances
> unfortunately - and I am going to put my foot down here - serial8 or 10
> is "fake" anyways - rationalization can be made in different ways. So
> pushing for one over the other is not something I will entertain.
>
>  From usage model point of view - serial0 will be the best candidate as
> console followed by serial2 (purely statistically speaking). In K3
> context, it is just serial2 in TI board ecosystem.
>
> That is a discussion for pros and cons - Open to hearing opinions.
> Unless I hear 1000% strong reasons with _backing data_ - not an
> subjective "it is correct thing to do" - why we'd want to move TI
> board ecosystem (including all the pains of bootloader combinations
> etc) switch over to ttyS0[1], having a mix and churn for the s/w
> ecosystem of having to deal with ttyS0 and ttyS2 nodes in arm64/dts/ti
> at least for the TI board ecosystem.. I cant see why i want to put the
> ecosystem through another churn on consoles.. But, fine, i will keep
> my ears open. If there are no strong arguments with _data_, then we
> stick with serial2 and y'all can curse me for another decade+ for that
> call :)
>
> [1] I have'nt forgotten ttyO2 to ttyS2 transition of OMAP
>   https://duckduckgo.com/?q=ttyO2+to+ttyS2+omap


Hi all,

I agree that there could be different standpoints on the aliasing of serial,
and the current way might not be the right way, but we have been using that
a quite long without any issues. (All K3 platforms use the same serial2 for
console).

IMO, if needed, the right way to change to other serial would be to:
1. Change all the serial aliases across K3 platforms, along with other
serial dependencies in userspace.
2. And have that as a separate patch series, i.e. a series that changes 
all the
incorrect aliases across K3 platforms. And this J784S4 patch series is 
not that
one.

Coming to a consensus would take time, lets do that separately and no 
point blocking
this series for it. All the previous platforms use the serial2 for 
console uart
and I will post my next series with the same. If there is a need to 
change the serial
aliasing, change it across all previous platforms in a separate patch 
series.

-- 
Thanks and regards,
Apurva Nandan,
Texas Instruments India.

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