linux-kernel - Re: [PATCH v3 13/14] serial: liteuart: add IRQ support for the TX path

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Date:   Tue, 15 Nov 2022 13:21:34 -0500
From:   "Gabriel L. Somlo" <gsomlo@...il.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-serial <linux-serial@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, kgugala@...micro.com,
        mholenko@...micro.com, joel@....id.au,
        david.abdurachmanov@...il.com, florent@...oy-digital.fr,
        geert@...ux-m68k.org
Subject: Re: [PATCH v3 13/14] serial: liteuart: add IRQ support for the TX
 path

On Tue, Nov 15, 2022 at 07:30:09PM +0200, Ilpo Järvinen wrote:
> On Tue, 15 Nov 2022, Gabriel L. Somlo wrote:
> 
> > On Tue, Nov 15, 2022 at 06:14:50PM +0200, Ilpo Järvinen wrote:
> > > On Sat, 12 Nov 2022, Gabriel Somlo wrote:
> > > 
> > > > Modify the TX path to operate in an IRQ-compatible way, while
> > > > maintaining support for polling mode via the poll timer.
> > > > 
> > > > Signed-off-by: Gabriel Somlo <gsomlo@...il.com>
> > > > ---
> > > >  drivers/tty/serial/liteuart.c | 67 ++++++++++++++++++++++++-----------
> > > >  1 file changed, 47 insertions(+), 20 deletions(-)
> > > > 
> > > > diff --git a/drivers/tty/serial/liteuart.c b/drivers/tty/serial/liteuart.c
> > > > index e30adb30277f..307c27398e30 100644
> > > > --- a/drivers/tty/serial/liteuart.c
> > > > +++ b/drivers/tty/serial/liteuart.c
> 
> > > > +	if (port->irq) {
> > > > +		u8 irq_mask = litex_read8(port->membase + OFF_EV_ENABLE);
> > > > +		litex_write8(port->membase + OFF_EV_ENABLE, irq_mask & ~EV_TX);
> > > 
> > > If you put irq_mask into liteuart_port you wouldn't need to read it 
> > > back here?
> > 
> > So, instead of `bool poll_tx_started` I should just keep a copy of the
> > irq_mask there, and take `&port->lock` whenever I need to *both* update
> > the mask *and* write it out to the actual device register?
> 
> I was mostly thinking of storing EV_RX there but then it could be derived 
> from port->irq that is checked by all paths already.

I'm a bit confused here -- the reason I was reading in irq_mask from
the mmio port and then flipping the EV_TX bit before writing it back
out was to preserve the current value of the EV_RX bit in that
register, whatever it may happen to be at the time. If I shadowed the
entire register value (with both EV_RX and EV_TX bits reflecting their
current value), I could get away with only ever *writing* the MMIO
register each time the shadow register value is modified (one or both
of the bits are flipped), and only when port->irq is non-zero (i.e.,
the shadow register would work for polling-mode also).

I think that's how altera_uart.c is doing it (I've been using that as
a source of inspiration).

I thought that's what you were suggesting earlie, but based on your
response above I'm no longer sure I follow.

> > > > +	if (unlikely(port->x_char)) {
> > > > +		litex_write8(port->membase + OFF_RXTX, port->x_char);
> > > > +		port->x_char = 0;
> > > > +		port->icount.tx++;
> > > > +		return;
> > > > +	}
> > > > +
> > > > +	while (!litex_read8(port->membase + OFF_TXFULL)) {
> > > > +		if (xmit->head == xmit->tail)
> > > 
> > > There exists a helper for this condition.
> > 
> > Is that in the released linus tree, or still only in tty-next?
> 
> uart_circ_empty() has been around for ages.

Oh, I misunderstood what you meant here originally -- just use
`uart_circ_empty()` like I'm already doing elsewhere in the code, got
it!

> > > > +	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
> > > > +		uart_write_wakeup(port);
> > > > +
> > > > +	if (uart_circ_empty(xmit))
> > > > +		liteuart_stop_tx(port);
> > > > +}
> > > 
> > > You might want to check if you can generate this whole function with 
> > > Jiri's tx helpers (IIRC, they're only in tty-next tree currently).
> > 
> > Looks like I should switch to tty-next for this whole series, which
> > makes sense, since it's a tty I'm working on :)
> > 
> > I'll rebase on top of that before I send out v4, hopefully later this
> > afternoon.
> 
> Ok.
> 
> As I now looked it up, Jiri's tx helpers is 
> 8275b48b278096edc1e3ea5aa9cf946a10022f79 and you'll find some example 
> conversions in the changes following it.

I'll check that out and follow the examples.

Thanks again,
--G