linux-kernel - Re: [PATCH v2 4/7] serial: qcom-geni: fix receiver enable

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Message-ID: <ZwaQbcuIPrR9HwKi@hovoldconsulting.com>
Date: Wed, 9 Oct 2024 16:17:17 +0200
From: Johan Hovold <johan@...nel.org>
To: Doug Anderson <dianders@...omium.org>
Cc: Johan Hovold <johan+linaro@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konradybcio@...nel.org>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org, stable@...r.kernel.org,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 4/7] serial: qcom-geni: fix receiver enable

On Thu, Oct 03, 2024 at 01:10:47PM -0700, Doug Anderson wrote:
> On Tue, Oct 1, 2024 at 5:51 AM Johan Hovold <johan+linaro@...nel.org> wrote:

> > @@ -1179,6 +1179,11 @@ static int qcom_geni_serial_startup(struct uart_port *uport)
> >                 if (ret)
> >                         return ret;
> >         }
> > +
> > +       uart_port_lock_irq(uport);
> > +       qcom_geni_serial_start_rx(uport);
> > +       uart_port_unlock_irq(uport);
> 
> I _think_ you don't need the locking here. The documentation for the
> "startup" callback say:
> 
>  * Interrupts: globally disabled.

Heh, yeah, that comment dates back to 2002 and probably wasn't even
correct back then.

This function is called with the port mutex held (and interrupts
enabled), and I need to take the port lock to serialise against the
console code.

> Other than that, this looks reasonable to me. I seem to recall
> previous discussions where _someone_ was relying on the
> qcom_geni_serial_start_rx() at the end of termios for some reason
> (which always felt like a bad design), but I can't find those old
> discussions. I suspect that the fact that you've added the start_rx in
> startup() is what we needed.

Yeah, I tried to find a reason for why things were done this way, but it
was probably just copied from the vendor driver. The hardware doesn't
seem to require stopping rx in set_termios() (and tx is not stopped
anyway), which could otherwise have been a reason.

Johan