Re: [PATCH v2] tty: serial: msm_serial: Fix flow control

[Andy Gross]

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@xxxxxxxxx>
> ---

Reviewed-by: Andy Gross <agross@xxxxxxxxxx>

Greg, can you pick this one up?

Thanks,
Andy