Re: [PATCH v8 06/11] serial: 8250_dw: derive RX trigger thresholds from the FIFO depth

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Thu, Nov 05, 2026 at 01:52:07PM -0700, James Hilliard wrote:
> DW_apb_uart uses receive thresholds of one byte, one quarter full,
> one half full and two bytes below full. The generic 16550A table describes
> these encodings only for a 16-byte FIFO. For example, the half-full setting
> on a 64-byte DW FIFO is reported as eight bytes instead of 32, and a sysfs
> request for 32 bytes selects the 62-byte encoding.
> 
> Build a per-port threshold table from the discovered FIFO depth in dwlib
> and install receive-trigger callbacks for CPR-described FIFOs, including
> LPSS ports. Keep the setup helper available to integrations which obtain
> the depth without probing the optional DW registers. Validate that each
> threshold is positive and representable before constructing the table.
> 
> Serialize FCR updates with termios and the port lock, acquire a runtime PM
> reference before accessing the device, and propagate resume errors without
> changing the shadow or hardware. Update only the trigger field so changing
> the threshold preserves the FIFO contents.
> 
> The FCR mapping is documented in the DW_apb_uart databook, section 6,
> FIFO Control Register. The existing default FCR encoding is unchanged.

...

> +static int dw8250_set_rxtrig(struct uart_port *p, u16 bytes)
> +{
> +	struct dw8250_port_data *pd = p->private_data;
> +	struct uart_8250_port *up = up_to_u8250p(p);
> +	int fcr, ret;
> +
> +	fcr = serial8250_bytes_to_fcr(pd->rxtrig_bytes, bytes);
> +	if (fcr < 0)
> +		return fcr;
> +
> +	guard(mutex)(&p->state->port.mutex);
> +
> +	PM_RUNTIME_ACQUIRE_IF_ENABLED(p->dev, pm);
> +	ret = PM_RUNTIME_ACQUIRE_ERR(&pm);
> +	if (ret)
> +		return ret;
> +
> +	guard(uart_port_lock_irqsave)(p);
> +
> +	up->fcr &= ~UART_FCR_TRIGGER_MASK;
> +	up->fcr |= fcr;

FIELD_MODIFY()

> +	/* Changing the trigger alone preserves the FIFO contents */
> +	serial_port_out(p, UART_FCR, up->fcr);
> +
> +	return 0;
> +}

-- 
With Best Regards,
Andy Shevchenko