Re: [PATCH 2/3] serial: 8250_dw: configure the Allwinner H616 UART

[Andy Shevchenko]

On Fri, Oct 23, 2026 at 01:33:50AM -0600, James Hilliard wrote:
> H616 has 256-byte transmit and receive FIFOs, but its optional DesignWare
> component parameter register reads as zero. Generic 16550 probing leaves
> the port using the 16-byte FIFO configuration and its trigger-level table.
> DMA attachment also depends on a FIFO size being available during probe,
> so these ports do not reach the existing 8250 DMA path.
> 
> Use a fixed port configuration for the H616 compatible, with a 256-byte
> transmit load and the documented receive thresholds of 1, 64, 128 and 254
> bytes. Select the quarter-full receive trigger by default.
> 
> Skip optional-register probing for this fixed configuration. H616 has
> debug-divisor and RS485 control registers at the offsets used to detect
> DesignWare hardware RS485 and fractional-divisor support. Probing these
> offsets writes unrelated registers and can select incompatible callbacks.
> Keep the BUSY-safe integer divisor hook and software RS485 handling.
> 
> Use the existing SKIP_SET_RATE quirk to keep the UART clock enabled during
> termios changes. Its clock is a gate on the shared APB2 source, rather than
> an independent baud-rate generator.
> 
> Attach DMA from the fixed configuration before registering the port. The
> 8250 core installs its FIFO defaults during registration, after the probe's
> FIFO-size test. Configure single-byte RX and TX bursts because the
> peripheral request can correspond to just one character. The existing
> startup path requests the firmware-described channels and falls back to
> PIO when they are unavailable or the port is a kernel console.

...

>  	/* Internal 8250 only */
>  	UART_PORT_AIROHA		= 124,
>  	UART_PORT_AIROHA_HS		= 125,
> +	UART_PORT_DW_APB_256	= 126,

Why? Besides the poorly chosen name, what prevents us from modifying the
corresponding parameters at probe time by leaving the one of the existing
types?

...

>  struct dw8250_platform_data {
>  	u8 usr_reg;
>  	u32 cpr_value;
> +	/* Use a fixed configuration instead of probing optional registers. */
> +	unsigned int type;
> +	unsigned int dma_maxburst;

This sounds like a hack.  Why do we need type here? The address of platform
data is enough, no?

>  	unsigned int quirks;

-- 
With Best Regards,
Andy Shevchenko