linux-kernel - Re: [PATCHv4 0/7] Serial port generic PM to fix 8250 PM

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Message-ID: <YbGyNW2EQlA/+VIg@atomide.com>
Date:   Thu, 9 Dec 2021 09:37:25 +0200
From:   Tony Lindgren <tony@...mide.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...el.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-serial@...r.kernel.org, linux-omap@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCHv4 0/7] Serial port generic PM to fix 8250 PM

* Johan Hovold <johan@...nel.org> [211130 10:03]:
> Specifically, it looks like tx can still stall indefinitely if the
> autosuspend timer fires. This can happen at low baud rates and also when
> using flow control.

Yeah the TX part is still problematic. Note that this is purely because
of current Linux serial layers implementation, and not because of any
hardware reasons.

Even after this series we still rely on serial8250_rpm_get_tx() and
serial8250_rpm_put_tx() to decipher if we can idle the port..

If anybody has good ideas where we can add the serial core TX related
paired runtime PM calls please let me know :)

For TX DMA, we should not do runtime PM put until at the DMA callback
function when completed.

Regards,

Tony