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

[Matthias Kaehlcke <mka@chromium.org>]

On Thu, Jan 10, 2019 at 08:22:12PM +0530, Balakrishna Godavarthi wrote:
> Hi Johan,
> 
> On 2019-01-10 20:09, Johan Hovold wrote:
> > On Thu, Jan 10, 2019 at 08:04:12PM +0530, Balakrishna Godavarthi wrote:
> > > Hi Johan,
> > > 
> > > On 2019-01-09 20:22, Johan Hovold wrote:
> > > > On Thu, Dec 20, 2018 at 08:16:36PM +0530, Balakrishna Godavarthi wrote:
> > > >> This patch will help to stop frame reassembly errors while changing
> > > >> the baudrate. This is because host send a change baudrate request
> > > >> command to the chip with 115200 bps, Whereas chip will change their
> > > >> UART clocks to the enable for new baudrate and sends the response
> > > >> for the change request command with newer baudrate, On host side
> > > >> we are still operating in 115200 bps which results of reading garbage
> > > >> data. Here we are pulling RTS line, so that chip we will wait to send
> > > >> data
> > > >> to host until host change its baudrate.
> > 
> > > >> +		/* Deassert RTS while changing the baudrate of chip and host.
> > > >> +		 * This will prevent chip from transmitting its response with
> > > >> +		 * the new baudrate while the host port is still operating at
> > > >> +		 * the old speed.
> > > >> +		 */
> > > >> +		qcadev = serdev_device_get_drvdata(hu->serdev);
> > > >> +		if (qcadev->btsoc_type == QCA_WCN3990)
> > > >> +			serdev_device_set_rts(hu->serdev, false);
> > > >> +
> > > >
> > > > This may not do what you want unless you also disable hardware flow
> > > > control.
> > 
> > > Here my requirement here is to block the chip to send its data before
> > > HOST changes it is baudrate. So if i disable flow control lines of
> > > HOST which will be in low state.  so that the chip will send it data
> > > before HOST change the baudrate of HOST. which results in frame
> > > reassembly error.
> > 
> > Not sure I understand what you're trying to say above. My point is that
> > you cannot reliable control RTS when you have automatic flow control
> > enabled (i.e. it is managed by hardware and it's state reflects whether
> > there's room in the UART receive FIFO).
> > 
> > Johan
> 
> [Bala]: Yes i got your point, but our driver

I suppose with "our driver" you refer to a Qualcomm UART driver like
qcom_geni_serial.c. Unless the Bluetooth controller is really tied to
some specific SoC (e.g. because it is on-chip) you shouldn't make
assumptions about the UART driver or hardware beyond standard
behavior.

But even if we assume that the driver you mention is used, I think you
are rather confirming Johan's concern than dispersing it:

> will not support automatic flow control (based on the FIFO status)
> unless we explicitly enabled via software. i.e. if we enable the
> flow, hardware will look for it else it will not looks for CTS or
> RTS Line.

So we agree that the UART hardware may change RTS if hardware flow
control is enabled?

static int qca_send_power_pulse(struct hci_uart *hu, u8 cmd)
{
  ...
  hci_uart_set_flow_control(hu, false);
  ...
}

I still find it utterly confusing that set_flow_control(false) enables
flow control, but that's what it does, hence after
qca_send_power_pulse() flow control is (re-)enabled.

So far I haven't seen problems with qcom_geni_serial.c overriding the
level set with serdev_device_set_rts(), but I tend to agree with Johan
that this could be a problem (if not with this UART (driver) then with
another). I'm not keen about adding more flow control on/off clutter,
but if that is needed for the driver to operate reliably across
platforms so be it.

Cheers

Matthias