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

Geert Uytterhoeven
2023-07-25T15:21:56Z

Hi Hugo,

On Tue, Jul 25, 2023 at 5:13 PM Hugo Villeneuve <hugo@hugovil.com> wrote:
> On Mon, 24 Jul 2023 18:08:00 +0200
> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Sat, Jul 22, 2023 at 8:18 PM Hugo Villeneuve <hugo@hugovil.com> wrote:
> > > From: Hugo Villeneuve <hvilleneuve@dimonoff.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@dimonoff.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@linux-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.
>
> Hi Geert,
> If this is desired, I think maybe it would be best to do it in a
> separate patch. Maybe there is a good reason why hd44780_common_home()

Sure, a separate patch would be fine.

> calls hd44780_common_gotoxy() instead of using Return Home command?

Looks like the Linux driver has always used goto 0/0 instead of Return Home.
Perhaps not all displays support that command? I know mine does.

Willy: do you remember?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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