linux-kernel - Re: [PATCH v4] tty: serial: don't do termios for BTIF

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Date:   Mon, 29 Jun 2020 11:29:27 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     sean.wang@...iatek.com, robh+dt@...nel.org, jslaby@...e.com,
        robert.jarzmik@...e.fr, arnd@...db.de, p.zabel@...gutronix.de,
        joel@....id.au, david@...hnology.com, jan.kiszka@...mens.com,
        heikki.krogerus@...ux.intel.com, hpeter@...il.com, vigneshr@...com,
        matthias.bgg@...il.com, tthayer@...nsource.altera.com,
        devicetree@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-serial@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Steven Liu <steven.liu@...iatek.com>,
        Ryder Lee <ryder.lee@...iatek.com>
Subject: Re: [PATCH v4] tty: serial: don't do termios for BTIF

On Sat, Jun 27, 2020 at 04:12:22PM +0200, Greg KH wrote:
> On Sat, Jun 20, 2020 at 03:59:14AM +0800, sean.wang@...iatek.com wrote:
> > From: Sean Wang <sean.wang@...iatek.com>
> > 
> > Bluetooth Interface (BTIF) is designed dedicatedly for MediaTek SOC with
> > BT in order to be instead of the UART interface between BT module and Host
> > CPU, and not exported to user space to access.
> > 
> > As the UART design, BTIF will be an APB slave and can transmit or receive
> > data by MCU access, but doesn't provide termios function like baudrate and
> > flow control setup.
> > 
> > Even LCR on offset 0xC that is just a FAKELCR
> > a. If FAKELCR[7] is equaled to 1, RBR(0x00), THR(0x00), IER(0x04)
> >    will not be readable/writable.
> > 
> > b. If FAKELCR is equaled to 0xBF, RBR(0x00), THR(0x00), IER(0x04),
> >    IIR(0x08), and LSR(0x14) will not be readable/writable.
> > 
> > So adding a new capability 'UART_CAP_NTIO' for the unusual unsupported
> > case.
> > 
> > The bluetooth driver would use BTIF device as a serdev. So the termios
> > still function would be called in kernelspace from ttyport_open in
> > drivers/tty/serdev/serdev-ttyprt.c.

> > +#define UART_CAP_NTIO	(1 << 18)	/* UART doesn't do termios */
> 
> Naming is hard.  I will never remember what "NTIO" is, how about we make
> it explicit:
> 	define UART_CAP_IGNORE_TERMIOS
> 
> And the _CAP_ name is getting out of hand, this isn't a "capability",
> it's a "quirk for this port" but that's a battle to worry about later...

For quirks we have a separate field in struct uart_port. Perhaps that can be used?

-- 
With Best Regards,
Andy Shevchenko