linux-kernel - Re: [PATCH tty-next v4 4/6] serial: 8250: Provide flag for IER toggling for RS485

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Message-ID: <84jzbaxg13.fsf@jogness.linutronix.de>
Date: Sun, 05 Jan 2025 01:32:00 +0106
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby
 <jirislaby@...nel.org>, Sergey Senozhatsky <senozhatsky@...omium.org>,
 Steven Rostedt <rostedt@...dmis.org>, Thomas Gleixner
 <tglx@...utronix.de>, Esben Haabendal <esben@...nix.com>,
 linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org, Florian
 Fainelli <florian.fainelli@...adcom.com>, Ray Jui <rjui@...adcom.com>,
 Scott Branden <sbranden@...adcom.com>, Broadcom internal kernel review
 list <bcm-kernel-feedback-list@...adcom.com>, Andy Shevchenko
 <andriy.shevchenko@...ux.intel.com>, Arnd Bergmann <arnd@...db.de>, Sunil
 V L <sunilvl@...tanamicro.com>, Matt Turner <mattst88@...il.com>, Stefan
 Wahren <wahrenst@....net>, Uwe Kleine-König
 <u.kleine-koenig@...libre.com>, Kevin Hilman <khilman@...libre.com>,
 Markus Schneider-Pargmann <msp@...libre.com>, Udit Kumar
 <u-kumar1@...com>, Griffin Kroah-Hartman <griffin@...ah.com>, Niklas
 Schnelle <schnelle@...ux.ibm.com>, Serge Semin <fancer.lancer@...il.com>,
 linux-rpi-kernel@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH tty-next v4 4/6] serial: 8250: Provide flag for IER
 toggling for RS485

On 2025-01-03, Petr Mladek <pmladek@...e.com> wrote:
> My understanding is that the nested IER manipulation does not
> harm.

This statement implies that it is OK for UART_IER_RLSI|UART_IER_RDI bits
to be set in UART_IER even though the console will write to UART_TX,
because the _nesting_ contexts would set those bits rather than
restoring the original value of 0x0.

I ran some tests and leaving these bits set during Tx does not appear to
cause an issue, but it is difficult to say because the context
interrupted by a nesting context will only print at most 1
character. Also, it is writing under spin_lock_irqsave() so that might
be masking any effects. Perhaps UART_IER is temporarly cleared because
of other bits that would cause problems during Tx?

I would need to create a specific test to investigate this
further. Regardless, it still is a cosmetic ugliness that bits are not
being properly restored, even if it turns out these particular bits are
not problematic during Tx.

As was with your LSR_DR comment, you are good at triggering fundamental
investigations into the history of the 8250 driver. ;-)

> All in all, the patch looks good to me.
>
> Reviewed-by: Petr Mladek <pmladek@...e.com>

Thanks for the review. I interpret it to mean that I do not need to make
any changes to this patch for v5.

John