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Date: Mon, 5 Feb 2024 08:56:45 +0100
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Subject: Re: [PATCH v1 tty] 8250: microchip: pci1xxxx: Add Burst mode
 transmission support in uart driver for reading from FIFO
Content-Language: en-US
To: Rengarajan S <rengarajan.s@microchip.com>,
 kumaravel.thiagarajan@microchip.com, tharunkumar.pasumarthi@microchip.com,
 gregkh@linuxfoundation.org, linux-serial@vger.kernel.org,
 linux-kernel@vger.kernel.org
Cc: UNGLinuxDriver@microchip.com
References: <20240125100006.153342-1-rengarajan.s@microchip.com>
From: Jiri Slaby <jirislaby@kernel.org>
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In-Reply-To: <20240125100006.153342-1-rengarajan.s@microchip.com>
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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@microchip.com>
> ---
>   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