linux-kernel - Re: [PATCH v2] auxdisplay: hd44780: move cursor home after clear display command

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Message-ID: <867cqm900q.fsf@davidreaver.com>
Date:   Wed, 26 Jul 2023 08:12:41 -0700
From:   David Reaver <me@...idreaver.com>
To:     Hugo Villeneuve <hugo@...ovil.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] auxdisplay: hd44780: move cursor home after clear
 display command


Hugo Villeneuve <hugo@...ovil.com> writes:

> From: Hugo Villeneuve <hvilleneuve@...onoff.com>
>
> The DISPLAY_CLEAR command on the NewHaven NHD-0220DZW-AG5 display
> does NOT change the DDRAM address to 00h (home position) like the
> standard Hitachi HD44780 controller. As a consequence, the starting
> position of the initial string LCD_INIT_TEXT is not guaranteed to be
> at 0,0 depending on where the cursor was before the DISPLAY_CLEAR
> command.
>
> Extract of DISPLAY_CLEAR command from datasheets of:
>
>     Hitachi HD44780:
>         ... It then sets DDRAM address 0 into the address counter...
>
>     NewHaven NHD-0220DZW-AG5 datasheet:
> 	... This instruction does not change the DDRAM Address
>
> Move the cursor home after sending DISPLAY_CLEAR command to support
> non-standard LCDs.
>
> Signed-off-by: Hugo Villeneuve <hvilleneuve@...onoff.com>
> ---
>  drivers/auxdisplay/hd44780_common.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/auxdisplay/hd44780_common.c b/drivers/auxdisplay/hd44780_common.c
> index 3934c2eebf33..7cbf375b0fa5 100644
> --- a/drivers/auxdisplay/hd44780_common.c
> +++ b/drivers/auxdisplay/hd44780_common.c
> @@ -82,7 +82,15 @@ int hd44780_common_clear_display(struct charlcd *lcd)
>  	hdc->write_cmd(hdc, LCD_CMD_DISPLAY_CLEAR);
>  	/* datasheet says to wait 1,64 milliseconds */
>  	long_sleep(2);
> -	return 0;
> +
> +	/*
> +	 * The Hitachi HD44780 controller (and compatible ones) reset the DDRAM
> +	 * address when executing the DISPLAY_CLEAR command, thus the
> +	 * following call is not required. However, other controllers do not
> +	 * (e.g. NewHaven NHD-0220DZW-AG5), thus move the cursor to home
> +	 * unconditionally to support both.
> +	 */
> +	return hd44780_common_home(lcd);
>  }
>  EXPORT_SYMBOL_GPL(hd44780_common_clear_display);
>
>
> base-commit: d192f5382581d972c4ae1b4d72e0b59b34cadeb9

I tested this on a 16x2 HD44780 on my BeagleBone Black with:

    $ printf '\f' > /dev/lcd
    $ printf 'Hello\nWorld!\n' > /dev/lcd
    $ printf '\x1b[LR' > /dev/lcd
    $ printf '\x1b[LR' > /dev/lcd
    $ printf '\x1b[LR' > /dev/lcd
    $ printf '\f' > /dev/lcd
    $ printf 'Goodbye\nWorld!\n' > /dev/lcd

As expected, "Goodbye World!" was correctly placed left-aligned on the
display, split over both lines.

Tested-by: David Reaver <me@...idreaver.com>