linux-kernel - Re: [PATCH v2 2/7] serial: 8250_dw: Avoid unnecessary LCR writes

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Message-ID: <aXoQwQnt4CYmeElZ@smile.fi.intel.com>
Date: Wed, 28 Jan 2026 15:36:01 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>, linux-serial@...r.kernel.org,
	qianfan Zhao <qianfanguijin@....com>,
	Adriana Nicolae <adriana@...sta.com>, linux-kernel@...r.kernel.org,
	"Bandal, Shankar" <shankar.bandal@...el.com>,
	"Murthy, Shanth" <shanth.murthy@...el.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/7] serial: 8250_dw: Avoid unnecessary LCR writes

On Wed, Jan 28, 2026 at 12:52:56PM +0200, Ilpo Järvinen wrote:
> When DW UART is configured with BUSY flag, LCR writes may not always
> succeed which can make any LCR write complex and very expensive.
> Performing write directly can trigger IRQ and the driver has to perform
> complex and distruptive sequence while retrying the write.
> 
> Therefore, it's better to avoid doing LCR write that would not change
> the value of the LCR register. Add LCR write avoidance code into the
> 8250_dw driver's .serial_out() functions.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

...

> +static bool dw8250_can_skip_reg_write(struct uart_port *p, unsigned int offset, u32 value)
> +{
> +	struct dw8250_data *d = to_dw8250_data(p->private_data);
> +	u32 lcr;
> +
> +	if (offset != UART_LCR || d->uart_16550_compatible)
> +		return false;
> +
> +	lcr = serial_port_in(p, offset);
> +	return lcr == value;

I don't expect the side-effects, but strictly speaking this compares
the 32-bit values, while in some callbacks the only part of that
is being used (often lowest 8 bits).

> +}

-- 
With Best Regards,
Andy Shevchenko