Re: [PATCH v1 tty] 8250: microchip: pci1xxxx: Add Burst mode transmission support in uart driver for reading from FIFO

[Rengarajan.S]

Hi Jiri Slaby,

The patch has been accepted and added in the tty-git tree and will be
merged during the next merged window. Should the changes be given as a
seperate incremental patch or should we re-submit this patch again. 

On Thu, 2024-02-08 at 02:49 +0000, Rengarajan S - I69107 wrote:
> Hi Jiri Slaby,
> 
> Thanks for reviewing the patch. Will address the comments in the next
> patch revision.
> 
> On Mon, 2024-02-05 at 08:56 +0100, Jiri Slaby wrote:
> > [Some people who received this message don't often get email from
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> > 
> > EXTERNAL EMAIL: Do not click links or open attachments unless you
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> > 
> > On 25. 01. 24, 11:00, Rengarajan S wrote:
> > > pci1xxxx_handle_irq reads the burst status and checks if the FIFO
> > > is empty and is ready to accept the incoming data. The handling
> > > is
> > > done in pci1xxxx_tx_burst where each transaction processes data
> > > in
> > > block of DWORDs, while any remaining bytes are processed
> > > individually,
> > > one byte at a time.
> > > 
> > > Signed-off-by: Rengarajan S <rengarajan.s@xxxxxxxxxxxxx>
> > > ---
> > >   drivers/tty/serial/8250/8250_pci1xxxx.c | 106
> > > ++++++++++++++++++++++++
> > >   1 file changed, 106 insertions(+)
> > > 
> > > diff --git a/drivers/tty/serial/8250/8250_pci1xxxx.c
> > > b/drivers/tty/serial/8250/8250_pci1xxxx.c
> > > index 558c4c7f3104..d53605bf908d 100644
> > > --- a/drivers/tty/serial/8250/8250_pci1xxxx.c
> > > +++ b/drivers/tty/serial/8250/8250_pci1xxxx.c
> > ...
> > > @@ -344,6 +348,105 @@ static void pci1xxxx_rx_burst(struct
> > > uart_port *port, u32 uart_status)
> > >       }
> > >   }
> > > 
> > > +static void pci1xxxx_process_write_data(struct uart_port *port,
> > > +                                     struct circ_buf *xmit,
> > > +                                     int *data_empty_count,
> > 
> > count is unsigned, right?
> > 
> > > +                                     u32 *valid_byte_count)
> > > +{
> > > +     u32 valid_burst_count = *valid_byte_count /
> > > UART_BURST_SIZE;
> > > +
> > > +     /*
> > > +      * Each transaction transfers data in DWORDs. If there are
> > > less than
> > > +      * four remaining valid_byte_count to transfer or if the
> > > circular
> > > +      * buffer has insufficient space for a DWORD, the data is
> > > transferred
> > > +      * one byte at a time.
> > > +      */
> > > +     while (valid_burst_count) {
> > > +             if (*data_empty_count - UART_BURST_SIZE < 0)
> > 
> > Huh?
> > 
> > *data_empty_count < UART_BURST_SIZE
> > 
> > instead?
> > 
> > > +                     break;
> > > +             if (xmit->tail > (UART_XMIT_SIZE -
> > > UART_BURST_SIZE))
> > 
> > Is this the same as easy to understand:
> > 
> > CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE) <
> > UART_BURST_SIZE
> > 
> > ?
> > 
> > > +                     break;
> > > +             writel(*(unsigned int *)&xmit->buf[xmit->tail],
> > > +                    port->membase + UART_TX_BURST_FIFO);
> > 
> > What about unaligned accesses?
> > 
> > And you really wanted to spell u32 explicitly, not uint.
> > 
> > > +             *valid_byte_count -= UART_BURST_SIZE;
> > > +             *data_empty_count -= UART_BURST_SIZE;
> > > +             valid_burst_count -= UART_BYTE_SIZE;
> > > +
> > > +             xmit->tail = (xmit->tail + UART_BURST_SIZE) &
> > > +                          (UART_XMIT_SIZE - 1);
> > 
> > uart_xmit_advance()
> > 
> > > +     }
> > > +
> > > +     while (*valid_byte_count) {
> > > +             if (*data_empty_count - UART_BYTE_SIZE < 0)
> > > +                     break;
> > > +             writeb(xmit->buf[xmit->tail], port->membase +
> > > +                    UART_TX_BYTE_FIFO);
> > > +             *data_empty_count -= UART_BYTE_SIZE;
> > > +             *valid_byte_count -= UART_BYTE_SIZE;
> > > +
> > > +             /*
> > > +              * When the tail of the circular buffer is reached,
> > > the next
> > > +              * byte is transferred to the beginning of the
> > > buffer.
> > > +              */
> > > +             xmit->tail = (xmit->tail + UART_BYTE_SIZE) &
> > > +                          (UART_XMIT_SIZE - 1);
> > 
> > uart_xmit_advance()
> > 
> > > +
> > > +             /*
> > > +              * If there are any pending burst count, data is
> > > handled by
> > > +              * transmitting DWORDs at a time.
> > > +              */
> > > +             if (valid_burst_count && (xmit->tail <
> > > +                (UART_XMIT_SIZE - UART_BURST_SIZE)))
> > > +                     break;
> > > +     }
> > > +}
> > 
> > This nested double loop is _really_ hard to follow. It's actually
> > terrible with cut & paste mistakes.
> > 
> > Could these all loops be simply replaced by something like this
> > pseudo
> > code (a single loop):
> > 
> > while (data_empty_count) {
> >    cnt = CIRC_CNT_TO_END();
> >    if (!cnt)
> >      break;
> >    if (cnt < UART_BURST_SIZE || (tail & 3)) { // is_unaligned()
> >      writeb();
> >      cnt = 1;
> >    } else {
> >      writel()
> >      cnt = UART_BURST_SIZE;
> >    }
> >    uart_xmit_advance(cnt);
> >    data_empty_count -= cnt;
> > }
> > 
> > ?
> > 
> > 
> > > +static void pci1xxxx_tx_burst(struct uart_port *port, u32
> > > uart_status)
> > > +{
> > > +     struct uart_8250_port *up = up_to_u8250p(port);
> > > +     u32 valid_byte_count;
> > > +     int data_empty_count;
> > > +     struct circ_buf *xmit;
> > > +
> > > +     xmit = &port->state->xmit;
> > > +
> > > +     if (port->x_char) {
> > > +             writeb(port->x_char, port->membase + UART_TX);
> > > +             port->icount.tx++;
> > > +             port->x_char = 0;
> > > +             return;
> > > +     }
> > > +
> > > +     if ((uart_tx_stopped(port)) || (uart_circ_empty(xmit))) {
> > > +             port->ops->stop_tx(port);
> > 
> > This looks weird standing here. You should handle this below along
> > with RPM.
> > 
> > > +     } else {
> > 
> > The condition should be IMO inverted with this block in its body:
> > 
> > > +             data_empty_count =
> > > (pci1xxxx_read_burst_status(port)
> > > &
> > > +                                 UART_BST_STAT_TX_COUNT_MASK) >>
> > > 8;
> > > +             do {
> > > +                     valid_byte_count =
> > > uart_circ_chars_pending(xmit);
> > > +
> > > +                     pci1xxxx_process_write_data(port, xmit,
> > > +                                                
> > > &data_empty_count,
> > > +                                                
> > > &valid_byte_count);
> > > +
> > > +                     port->icount.tx++;
> > 
> > Why do you increase the stats only once per burst? (Solved by
> > uart_xmit_advance() above.)
> > 
> > > +                     if (uart_circ_empty(xmit))
> > > +                             break;
> > > +             } while (data_empty_count && valid_byte_count);
> > > +     }
> > > +
> > > +     if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
> > > +             uart_write_wakeup(port);
> > > +
> > > +      /*
> > > +       * With RPM enabled, we have to wait until the FIFO is
> > > empty
> > > before
> > > +       * the HW can go idle. So we get here once again with
> > > empty
> > > FIFO and
> > > +       * disable the interrupt and RPM in __stop_tx()
> > > +       */
> > > +     if (uart_circ_empty(xmit) && !(up->capabilities &
> > > UART_CAP_RPM))
> > > +             port->ops->stop_tx(port);
> > 
> > I wonder why this driver needs this and others don't? Should they
> > be
> > fixed too?
> > 
> > > +}
> > > +
> > >   static int pci1xxxx_handle_irq(struct uart_port *port)
> > >   {
> > >       unsigned long flags;
> > > @@ -359,6 +462,9 @@ static int pci1xxxx_handle_irq(struct
> > > uart_port
> > > *port)
> > >       if (status & UART_BST_STAT_LSR_RX_MASK)
> > >               pci1xxxx_rx_burst(port, status);
> > > 
> > > +     if (status & UART_BST_STAT_LSR_THRE)
> > > +             pci1xxxx_tx_burst(port, status);
> > > +
> > >       spin_unlock_irqrestore(&port->lock, flags);
> > > 
> > >       return 1;
> > 
> > --
> > js
> > suse labs
> > 
>