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From:   Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Date:   Sun, 3 Nov 2019 14:51:17 -0700
Message-ID: <CAOCk7Nptm=Cz17FFKKvsgVxXRgJ-m9zK4RKysqhjb4cwPweSXg@mail.gmail.com>
Subject: Re: [PATCH v2] tty: serial: msm_serial: Fix flow control
To:     Andy Gross <agross@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Bjorn Andersson <bjorn.andersson@linaro.org>, jslaby@suse.com,
        MSM <linux-arm-msm@vger.kernel.org>,
        linux-serial@vger.kernel.org, lkml <linux-kernel@vger.kernel.org>
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On Sat, Oct 26, 2019 at 11:55 PM Andy Gross <agross@kernel.org> wrote:
>
> On Mon, Oct 21, 2019 at 08:46:16AM -0700, Jeffrey Hugo wrote:
> > hci_qca interfaces to the wcn3990 via a uart_dm on the msm8998 mtp and
> > Lenovo Miix 630 laptop.  As part of initializing the wcn3990, hci_qca
> > disables flow, configures the uart baudrate, and then reenables flow - at
> > which point an event is expected to be received over the uart from the
> > wcn3990.  It is observed that this event comes after the baudrate change
> > but before hci_qca re-enables flow. This is unexpected, and is a result of
> > msm_reset() being broken.
> >
> > According to the uart_dm hardware documentation, it is recommended that
> > automatic hardware flow control be enabled by setting RX_RDY_CTL.  Auto
> > hw flow control will manage RFR based on the configured watermark.  When
> > there is space to receive data, the hw will assert RFR.  When the watermark
> > is hit, the hw will de-assert RFR.
> >
> > The hardware documentation indicates that RFR can me manually managed via
> > CR when RX_RDY_CTL is not set.  SET_RFR asserts RFR, and RESET_RFR
> > de-asserts RFR.
> >
> > msm_reset() is broken because after resetting the hardware, it
> > unconditionally asserts RFR via SET_RFR.  This enables flow regardless of
> > the current configuration, and would undo a previous flow disable
> > operation.  It should instead de-assert RFR via RESET_RFR to block flow
> > until the hardware is reconfigured.  msm_serial should rely on the client
> > to specify that flow should be enabled, either via mctrl() or the termios
> > structure, and only assert RFR in response to those triggers.
> >
> > Fixes: 04896a77a97b ("msm_serial: serial driver for MSM7K onboard serial peripheral.")
> > Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
> > ---
>
> Reviewed-by: Andy Gross <agross@kernel.org>
>
> Greg, can you pick this one up?
>
> Thanks,
> Andy

Greg, will this be queued for 5.5?