linux-kernel - Re: [PATCH 2/2] serial: qcom_geni_serial: Always use 4 bytes per TX FIFO word

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Date:   Fri, 10 Jul 2020 12:05:01 -0700
From:   Evan Green <evgreen@...omium.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Akash Asthana <akashast@...eaurora.org>,
        Stephen Boyd <swboyd@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Jiri Slaby <jslaby@...e.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-serial@...r.kernel.org
Subject: Re: [PATCH 2/2] serial: qcom_geni_serial: Always use 4 bytes per TX
 FIFO word

On Fri, Jul 10, 2020 at 11:28 AM Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Fri, Jul 10, 2020 at 10:46 AM Evan Green <evgreen@...omium.org> wrote:
> >
> > On Fri, Jun 26, 2020 at 1:01 PM Douglas Anderson <dianders@...omium.org> wrote:
> > >
> > > The geni serial driver had a rule that we'd only use 1 byte per FIFO
> > > word for the TX FIFO if we were being used for the serial console.
> > > This is ugly and a bit of a pain.  It's not too hard to fix, so fix
> > > it.
> > >
> > > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > > ---
> > >
> > >  drivers/tty/serial/qcom_geni_serial.c | 57 +++++++++++++++++----------
> > >  1 file changed, 37 insertions(+), 20 deletions(-)
> > >
> > > diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> > > index 4610e391e886..583d903321b5 100644
> > > --- a/drivers/tty/serial/qcom_geni_serial.c
> > > +++ b/drivers/tty/serial/qcom_geni_serial.c
> > > @@ -103,12 +103,18 @@
> > >  #define DEFAULT_IO_MACRO_IO2_IO3_MASK          GENMASK(15, 4)
> > >  #define IO_MACRO_IO2_IO3_SWAP          0x4640
> > >
> > > +/* We always configure 4 bytes per FIFO word */
> > > +#define BYTES_PER_FIFO_WORD            4
> > > +
> > >  struct qcom_geni_private_data {
> > >         /* NOTE: earlycon port will have NULL here */
> > >         struct uart_driver *drv;
> > >
> > >         u32 poll_cached_bytes;
> > >         unsigned int poll_cached_bytes_cnt;
> > > +
> > > +       u32 write_cached_bytes;
> > > +       unsigned int write_cached_bytes_cnt;
> > >  };
> > >
> > >  struct qcom_geni_serial_port {
> > > @@ -121,8 +127,6 @@ struct qcom_geni_serial_port {
> > >         bool setup;
> > >         int (*handle_rx)(struct uart_port *uport, u32 bytes, bool drop);
> > >         unsigned int baud;
> > > -       unsigned int tx_bytes_pw;
> > > -       unsigned int rx_bytes_pw;
> > >         void *rx_fifo;
> > >         u32 loopback;
> > >         bool brk;
> > > @@ -390,13 +394,25 @@ static void qcom_geni_serial_poll_put_char(struct uart_port *uport,
> > >  #ifdef CONFIG_SERIAL_QCOM_GENI_CONSOLE
> > >  static void qcom_geni_serial_wr_char(struct uart_port *uport, int ch)
> > >  {
> > > -       writel(ch, uport->membase + SE_GENI_TX_FIFOn);
> > > +       struct qcom_geni_private_data *private_data = uport->private_data;
> > > +
> > > +       private_data->write_cached_bytes =
> > > +               (private_data->write_cached_bytes >> 8) | (ch << 24);
> > > +       private_data->write_cached_bytes_cnt++;
> > > +
> > > +       if (private_data->write_cached_bytes_cnt == BYTES_PER_FIFO_WORD) {
> > > +               writel(private_data->write_cached_bytes,
> > > +                      uport->membase + SE_GENI_TX_FIFOn);
> > > +               private_data->write_cached_bytes_cnt = 0;
> > > +       }
> > >  }
> > >
> > >  static void
> > >  __qcom_geni_serial_console_write(struct uart_port *uport, const char *s,
> > >                                  unsigned int count)
> > >  {
> > > +       struct qcom_geni_private_data *private_data = uport->private_data;
> > > +
> > >         int i;
> > >         u32 bytes_to_send = count;
> > >
> > > @@ -431,6 +447,15 @@ __qcom_geni_serial_console_write(struct uart_port *uport, const char *s,
> > >                                                         SE_GENI_M_IRQ_CLEAR);
> > >                 i += chars_to_write;
> > >         }
> > > +
> > > +       if (private_data->write_cached_bytes_cnt) {
> > > +               private_data->write_cached_bytes >>= BITS_PER_BYTE *
> > > +                       (BYTES_PER_FIFO_WORD - private_data->write_cached_bytes_cnt);
> > > +               writel(private_data->write_cached_bytes,
> > > +                      uport->membase + SE_GENI_TX_FIFOn);
> > > +               private_data->write_cached_bytes_cnt = 0;
> > > +       }
> >
> > How does this not end up sending stray zeros? In other words, how does
> > the hardware know which bytes of this word are valid?
>
> We told it how many bytes we wanted to send in
> qcom_geni_serial_setup_tx().  If the total number of bytes being sent
> is not a multiple of the FIFO word size then it knows that the last
> word will be a partial and it'll extract just the number of needed
> bytes out of it.

Ohh right. Sounds good.

Reviewed-by: Evan Green <evgreen@...omium.org>