Re: [PATCH v5 2/5] Bluetooth: hci_qca: Deassert RTS while baudrate change command

From: Balakrishna Godavarthi <bgodavar@codeaurora.org>
To: Matthias Kaehlcke <mka@chromium.org>
Cc: Johan Hovold <johan@kernel.org>,
	marcel@holtmann.org, johan.hedberg@gmail.com,
	linux-kernel@vger.kernel.org, linux-bluetooth@vger.kernel.org,
	hemantg@codeaurora.org, linux-arm-msm@vger.kernel.org,
	Johan Hovold <jhovold@gmail.com>
Subject: Re: [PATCH v5 2/5] Bluetooth: hci_qca: Deassert RTS while baudrate change command
Date: Wed, 23 Jan 2019 18:27:42 +0530	[thread overview]
Message-ID: <f5e3080235572c44fe1e8c0d4b2e5a97@codeaurora.org> (raw)
In-Reply-To: <20190122215330.GI261387@google.com>

Hi Matthias,

On 2019-01-23 03:23, Matthias Kaehlcke wrote:
> On Mon, Jan 21, 2019 at 08:11:39PM +0530, Balakrishna Godavarthi wrote:
>> Hi Matthias,
>> 
>> On 2019-01-19 06:01, Matthias Kaehlcke wrote:
>> > On Fri, Jan 18, 2019 at 10:44:16AM +0100, Johan Hovold wrote:
>> > > On Thu, Jan 17, 2019 at 09:21:09AM -0800, Matthias Kaehlcke wrote:
>> > >
>> > > > I observed that the qcom_geni_serial driver doesn't raise RTS with
>> > > > flow control disabled. It seems we have to investigate why that's the
>> > > > case. I agree that the driver should be platform agnostic.
>> > >
>> > > Sounds like a driver bug, unless the hardware is just "odd". The
>> > > driver implementation of this looks very non-standard judging from a
>> > > quick peek.
>> > >
>> > > In fact, qcom_geni_serial_get_mctrl() is currently a no-op if hardware
>> > > flow control is not enabled:
>> > >
>> > > 	if (uart_console(uport) || !uart_cts_enabled(uport))
>> > > 		return;
>> > >
>> > > Perhaps dropping the !uart_cts_enabled() test is sufficient.
>> >
>> > Thanks for taking a look Johan, that was indeed the problem (also
>> > in set_mctrl()). I posted a fix:
>> > https://lore.kernel.org/patchwork/patch/1033611/
>> >
>> > Balakrishna, the following (applied on top of your patch) works for me
>> > with the UART patch above:
>> >
>> 
>> [Bala]: will test and update BT patch accordingly.
> 
> Note that Johan pointed out that hci_uart_set_flow_control() deasserts
> RTS when flow control is disabled, so the _set_rts() calls can be
> removed. With that only a single action needs to be undone in case of
> an error, you can consider to keep the return instead of using the
> goto introduced by my patch.
> 

[Bala]: ok sure. will note this.

> Please keep/adapt the "Deassert RTS while changing the baudrate ..."
> comment to leave it clear to posterity why flow control is disabled
> during baudrate changes. It's fairly obvious once you understand the
> problem and that disabling flow control deasserts RTS, but it took us
> a while to get there, initially we only had a comment saying
> "disabling flow control is mandatory" (I recall I inquired about this
> multiple times during the initial review of the wcn3990 patches ;-)
> 
> Thanks
> 
> Matthias

-- 
Regards
Balakrishna.