Re: [PATCHv3 02/10] serdev: add serdev_device_wait_until_sent

[Sebastian Reichel <sre@kernel.org>]

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Hi,

On Mon, Apr 10, 2017 at 11:12:39AM -0500, Rob Herring wrote:
> On Mon, Apr 10, 2017 at 9:03 AM, Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> > On Mon, Apr 10, 2017 at 08:46:57AM -0500, Rob Herring wrote:
> >> On Sat, Apr 8, 2017 at 11:57 AM, Greg Kroah-Hartman
> >> <gregkh@linuxfoundation.org> wrote:
> >> > On Tue, Mar 28, 2017 at 05:59:31PM +0200, Sebastian Reichel wrote:
> >> >> Add method, which waits until the transmission buffer has been sent.
> >> >> Note, that the change in ttyport_write_wakeup is related, since
> >> >> tty_wait_until_sent will hang without that change.
> >> >>
> >> >> Acked-by: Rob Herring <robh@kernel.org>
> >> >> Acked-by: Pavel Machek <pavel@ucw.cz>
> >> >> Signed-off-by: Sebastian Reichel <sre@kernel.org>
> >> >> ---
> >> >> Changes since PATCHv2:
> >> >>  * Avoid goto in ttyport_write_wakeup
> >> >> ---
> >> >>  drivers/tty/serdev/core.c           | 11 +++++++++++
> >> >>  drivers/tty/serdev/serdev-ttyport.c | 18 ++++++++++++++----
> >> >>  include/linux/serdev.h              |  3 +++
> >> >>  3 files changed, 28 insertions(+), 4 deletions(-)
> >> >>
> >> >> diff --git a/drivers/tty/serdev/core.c b/drivers/tty/serdev/core.c
> >> >> index f4c6c90add78..a63b74031e22 100644
> >> >> --- a/drivers/tty/serdev/core.c
> >> >> +++ b/drivers/tty/serdev/core.c
> >> >> @@ -173,6 +173,17 @@ void serdev_device_set_flow_control(struct serdev_device *serdev, bool enable)
> >> >>  }
> >> >>  EXPORT_SYMBOL_GPL(serdev_device_set_flow_control);
> >> >>
> >> >> +void serdev_device_wait_until_sent(struct serdev_device *serdev, long timeout)
> >> >> +{
> >> >> +     struct serdev_controller *ctrl = serdev->ctrl;
> >> >> +
> >> >> +     if (!ctrl || !ctrl->ops->wait_until_sent)
> >> >> +             return;
> >> >> +
> >> >> +     ctrl->ops->wait_until_sent(ctrl, timeout);
> >> >> +}
> >> >> +EXPORT_SYMBOL_GPL(serdev_device_wait_until_sent);
> >> >
> >> > Is this still needed now that we have serdev_device_write() with an
> >> > unlimited timeout available?
> >>
> >> Yes, because only this waits until the data is on the wire.
> >
> > What "wire" is that?  The serial wire?  How do you know this?  Many usb
> > to serial devices have no way to determine this, given that there is
> > another uart hanging off of the end of a USB connection.
> 
> Okay, maybe it's just out of linux s/w buffers for h/w which you don't
> know. It is the same semantics as tty_wait_until_sent which is
> documented as: "Wait for characters pending in a tty driver to hit the
> wire, or for a timeout to occur (eg due to flow control)"

For embedded h/w it usually means the serial wire. tty_wait_until_sent()
first waits for the tty buffer to be empty and then calls wait_until_sent()
in the driver providing the tty. In case of serial-core that is
implemented by uart_wait_until_sent(), which waits for the serial
driver's tx_empty() operation becoming true (grepping for
".tx_empty" returned 81 hits for me). Also at least some of the usb to
serial adapters seem to support this using usb_serial_generic_wait_until_sent()
and ".tx_empty":

$ git grep "\.tx_empty"
cp210x.c:       .tx_empty               = cp210x_tx_empty,
f81534.c:       .tx_empty =             f81534_tx_empty,
ftdi_sio.c:     .tx_empty =             ftdi_tx_empty,
io_ti.c:        .tx_empty               = edge_tx_empty,
io_ti.c:        .tx_empty               = edge_tx_empty,
mxuport.c:      .tx_empty               = mxuport_tx_empty,
ti_usb_3410_5052.c:     .tx_empty               = ti_tx_empty,
ti_usb_3410_5052.c:     .tx_empty               = ti_tx_empty,

The other ones will only wait for empty s/w buffer, but that's
already the case for tty_wait_until_sent(), so IMHO a different
problem.

-- Sebastian

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