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 10:38:50 -0700
From:   Evan Green <evgreen@...omium.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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@...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, 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?