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

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Date:   Mon, 24 Jul 2023 18:08:00 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Hugo Villeneuve <hugo@...ovil.com>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Hugo Villeneuve <hvilleneuve@...onoff.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] auxdisplay: hd44780: move cursor home after clear
 display command

Hi Hugo,

On Sat, Jul 22, 2023 at 8:18 PM Hugo Villeneuve <hugo@...ovil.com> wrote:
> 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>

Thanks for your patch!

> --- 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);

Sorry, I haven't tested your patch yet, as my HD44780 display is
currently not connected to any system.

Your patch LGTM, so
Reviewed-by: Geert Uytterhoeven <geert@...ux-m68k.org>

Note that hd44780_common_home() calls hd44780_common_gotoxy().
According to the documentation, both HD44780U and NHD-0220DZW-AG5
support the Return Home command (0x02), so that could be a good
alternative.

>  }
>  EXPORT_SYMBOL_GPL(hd44780_common_clear_display);

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds