Re: [PATCH v4 1/2] serial: Keep rs485 settings for devices without firmware node

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

On Thu, Oct 23, 2025 at 05:12:28PM +0200, Gerhard Engleder wrote:
> 
> Commit fe7f0fa43cef ("serial: 8250: Support rs485 devicetree properties")
> retrieves rs485 properties for 8250 drivers. These properties are read
> from the firmware node of the device within uart_get_rs485_mode(). If the
> firmware node does not exist, then the rs485 flags are still reset. Thus,
> 8250 driver cannot set rs485 flags to enable a defined rs485 mode during
> driver loading. This is no problem so far, as no 8250 driver sets the
> rs485 flags.
> 
> The default rs485 mode can also be set by firmware nodes. But for some
> devices a firmware node does not exist. E.g., for a PCIe based serial
> interface on x86 no device tree is available and the ACPI information of
> the BIOS often cannot by modified.

While the code is okay per se, the above statement is not fully true.
We specifically have device properties and software nodes to provide
the missing bits.

> In this case it shall be possible,
> that a driver works out of the box by setting a reasonable default rs485
> mode.
> 
> If no firmware node exists, then it should be possible for the driver to
> set a reasonable default rs485 mode. Therefore, reset rs485 flags only
> if a firmware node exists.

-- 
With Best Regards,
Andy Shevchenko