linux-kernel - Re: [PATCH v2 28/32] auxdisplay: hd44780: Remove clear

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Date:   Tue, 22 Sep 2020 10:49:12 +0200
From:   Lars Poeschel <poeschel@...onage.de>
To:     Willy Tarreau <w@....eu>
Cc:     Miguel Ojeda Sandonis <miguel.ojeda.sandonis@...il.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 28/32] auxdisplay: hd44780: Remove clear_fast

Hi Willy,

On Tue, Sep 22, 2020 at 07:22:27AM +0200, Willy Tarreau wrote:
> Hi Lars,
> 
> On Mon, Sep 21, 2020 at 04:46:40PM +0200, poeschel@...onage.de wrote:
> > From: Lars Poeschel <poeschel@...onage.de>
> > 
> > We remove the hd44780_clear_fast (display) clear implementation. charlcd
> > will fall back to use hd44780_common_clear_display then, which is much
> > much faster.
> 
> I might have got confused, but it looks to me like patches 27 and 28
> basically undo patch 26: in 26 you moved code to hd44780 and wrote a
> fallback, just to later delete that code.

To be honest: I also got confused by this whole clear code and why are
there 3 different clear variants. ;-)
The reason why I did it this way is to show what happened to the
original code. The original code has a fallback that kind of does the
same like my naive implementation but it uses the fact, that on hd44780
displays the underlying buffer can be accessed in a linear manner to
speed things up a little. This then also clears the not visible area of
the buffer (which my naive implementation does not). To become
independent of hd44780 code at this point I had to do something.
I opted to for this naive implementation as a replacement for the old
optimized clear loop. The fallback was already there.
And then I did a separate step to remove it because I found it a bit
suboptimal. All current users have the clear command...

> I seem to remember that the reason for the clear_fast() implementation
> is that the default clear_display() is quite slow for small displays,
> compared to what can be achieved by just filling the display with spaces
> (in the order of tens of milliseconds vs hundreds of microseconds). But
> I could be mistaken given how old this is, so please take my comment
> with a grain of salt.

... well, and what the hd44780 controller does when it executes the
clear command is also writing a space character to all character
locations (also to the not visible ones). Probably very much the same
than the original fallback implementation.
For me issuing the clear command is faster by at least one order!
I have one of these cheap china displays with 4x20 characters. The flow
is like this: i2c -> gpio -> hd44780 in 4 bit mode. And gpio is issuing
a i2c request for EVERY SINGLE gpio. This is slow as hell. But it works.
:-)
As I said I also was a bit confused by the different clean
implementations, but the only real user of the clear_fast is the panel
driver. The hd44780 one I use did not provide a clear_fast.

So I would opt for the way I proposed in the patch series with the clear
command instead of the loop. And let the panel driver use its optimized
clear.

I am open about squashing things together if that is wanted, so we don't
have the step inbetween with the naive implementation and to not
confuse people. ;-)

Regards,
Lars