linux-kernel - Re: [PATCH 0/7] serial: 8250: Add rs485 emulation to 8250

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Date:   Wed, 18 Mar 2020 19:49:08 +0100
From:   Heiko Stübner <heiko@...ech.de>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        gregkh@...uxfoundation.org, jslaby@...e.com,
        matwey.kornilov@...il.com, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] serial: 8250: Add rs485 emulation to 8250_dw

Hi,

Am Mittwoch, 18. März 2020, 16:37:54 CET schrieb Lukas Wunner:
> On Wed, Mar 18, 2020 at 04:43:20PM +0200, Andy Shevchenko wrote:
> > On Wed, Mar 18, 2020 at 03:26:33PM +0100, Heiko Stuebner wrote:
> > > This series tries to revive the work of Giulio Benetti from 2018 [0]
> > > which seemed to have stalled at that time.
> 
> Oh dear. :-(  This needs a rebase on current tty-next.

Looking at tty-next I notice that you're right. When I started working
on this I only found the stuff from 2018 I linked to but didn't imagine
that in that exact moment someone else would also work on that area.

So no worries, I'll adapt my code ;-)


> Patch [7/7] is already in tty-next as commit fe7f0fa43cef ("serial:
> 8250: Support rs485 devicetree properties").
>
> Patch [4/7] likewise.  Note that it's not safe to call ->rs485_config()
> already in serial8250_register_8250_port() if the driver uses UPF_IOREMAP
> because ioremapping happens later via serial8250_config_port() ->
> serial8250_request_std_resource(), so you'll get an oops for those
> drivers when deasserting RTS early on.  Been there... :-(
> 
> Patch [6/7]:  Ugh, another duplication of the ->rs485_config() callback.
> Just use the generic one introduced by commit 283e096ffb70 ("serial:
> 8250: Deduplicate ->rs485_config() callback") if possible.
> 
> The other patches appear to handle chip-specific needs.  It's now
> possible to implement these in ->rs485_start_tx() and ->rs485_stop_tx()
> hooks, as introduced by commit 058bc104f7ca ("serial: 8250: Generalize
> rs485 software emulation").  Refer to commit f93bf7589114 ("serial:
> 8250_bcm2835aux: Support rs485 software emulation") for an example.

Thanks for the pointers and also doing all that ground work :-)


> The DTR-for-RE thing seems a bit nonstandard, I'm not sure if this is
> eligible for mainline or if it's something that should be kept in your
> downstream tree.  But no harm in submitting it to the list.

I'm fine either way - maybe I also get a pointer on what may be a better
approach ;-)

At least DTR as "Data Terminal Ready" did sound somewhat matching for
the "Receive Enable" of RS485 (and it's also the only other output pin
in the mctrl gpio list).


Heiko