linux-kernel - Re: [PATCH] Logitech G13 driver (fixed cc list --- ignore others)

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Date:	Thu, 14 Jan 2010 23:34:21 +0100
From:	Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:	"Rick L. Vinyard, Jr." <rvinyard@...nmsu.edu>
Cc:	Jaya Kumar <jayakumar.lkml@...il.com>, Pavel Machek <pavel@....cz>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
	krzysztof.h1@...pl, Andrew Morton <akpm@...ux-foundation.org>,
	linux-usb@...r.kernel.org, Oliver Neukum <oliver@...kum.org>,
	linux-input@...r.kernel.org
Subject: Re: [PATCH] Logitech G13 driver (fixed cc list --- ignore others)

On Thu, Jan 14, 2010 at 10:48 PM, Rick L. Vinyard, Jr.
<rvinyard@...nmsu.edu> wrote:
> Miguel Ojeda wrote:
>> On Tue, Jan 5, 2010 at 1:14 AM, Jaya Kumar <jayakumar.lkml@...il.com>
>> wrote:
>>> On Tue, Jan 5, 2010 at 6:48 AM, Pavel Machek <pavel@....cz> wrote:
>>>>
>>>> Ok, but I guess you should cc auxdisplay people in future.
>>>
>>> Hi Pavel,
>>>
>>> I just looked at the drivers/auxdisplay directory and got a bit
>>> confused. The reason I got confused is because auxdisplay is actually
>>> an fbdev driver but it is outside of the drivers/video directory. It
>>> looks like there has only been 1 commit and that was for the Samsung
>>> KS0108 controller. It also sort of uses platform support but the way
>>> it is abstracted is odd to my thinking. The controller is ks0108, so
>>> in my mind that would be the code that would be platform independent,
>>> and then that would use a board specific IO driver to push data (eg:
>>> parport or gpio or usb). I think in the long term it would probably
>>> make sense to write a cleaner approach with a drivers/video/ks0108.c
>>> which is cleanly platform independent (and back within fbdev proper)
>>> and then a board specific driver in the appropriate location that
>>> handles the IO.
>>
>> I wrote long ago the driver(s) and people that reviewed it thought it
>> was better to keep it outside. I think that if someone else is going
>> to need ks0108, then I agree: we should write a independent driver.
>>
>> It should not be hard, it is an easy controller to play with and the
>> code is already there. I would try to do it; however, I am not sure if
>> I would be the most appropriate person to code such generic driver, as
>> I know almost nothing about all drivers/video/* stuff and the ways of
>> making it truly generic for future video/ users. Still, I will help
>> gladly.
>>
>
> When I started to look at writing the G13 framebuffer the first code I
> looked at was the cfag12864b, and started off trying to adapt it.
>

I hope it was useful, at least at first. : )

> However, as I was digging through the video/* directory looking for
> something (I forget now what) I came across the hecubafb and patterned the
> G13 after it instead.
>
> In moving between the two, the biggest difference was that I was able to
> strip out alot of the workqueue code you had since all that was provided
> by defio. Otherwise, the general structure was almost identical.
>
> In particular, what would change is the lower half of cfag12864b.c and you
> would be able to eliminate almost everything from the /* Update work */
> and below comment with the exception of cfag12864b_update().
>
> cfag12864b_update() would become almost analogous to the g13_fb_update() I
> have in the G13 driver which is triggered by the deferred_io member of the
> fb_deferred_io structure.
>
> You would have something like:
>
> /* Callback from deferred IO workqueue */
> static void cfag12864b_deferred_io(struct fb_info *info, struct list_head
> *pagelist)
> {
>        cfag12864b_update(info->par);
> }
>
> static struct fb_deferred_io cfag12864b_defio = {
>        .delay = HZ / CFAG12864B_UPDATE_RATE_DEFAULT,
>        .deferred_io = cfag12864b_deferred_io,
> };
>

Thank you for the analysis of cfag12864b. See below.

>
> The other major change is that you could eliminate the periodic memcmp()
> to see if the buffer has change since the deferred_io is only going to
> trigger on a page write fault.

Yeah, I admit the memcmp() is pretty ugly knowing about deferred_io,
which I did not. It is strange that anyone pointed it out long before,
is it new? Are there any known drawbacks?

>
> But, that isn't a major change in the code... only in performance.
>

So less code and greater performance. That sounds like a winning deal!

About ks0108, have you got any thoughts on how to write a generic
driver? Do you need something special about ks0108? I only needed raw
output operations so I just implemented that. Also, cfag12864b uses
two ks0108 controllers and I suppose other LCD's use many more, so
there are many points that may need a "research".

Miguel Ojeda.

> ---
>
> Rick
>
>
>
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