linux-kernel - Re: [RFC PATCH] video: Use fb_sys_write rather than open-coding in drivers

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Date:	Wed, 12 Feb 2014 08:54:40 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Ryan Mallon <rmallon@...il.com>
CC:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] video: Use fb_sys_write rather than open-coding in
 drivers

On 11/02/14 21:07, Ryan Mallon wrote:
> On 12/02/14 03:06, Tomi Valkeinen wrote:
> 
>> On 20/09/13 10:06, Ryan Mallon wrote:
>>> Several video drivers open code the fb_write write function with code
>>> which is very similar to fb_sys_write. Replace the open code versions
>>> with calls to fb_sys_write. An fb_sync callback is added to each of
>>> the drivers.
>>>     
>>> Signed-off-by: Ryan Mallon <rmallon@...il.com>
>>> ---
>>
>> Doesn't this change the behavior so that fb_write does no longer update
>> the display, but fb_sync does? I don't think fb_sync is even meant to
>> update the display, it's meant to wait for an update to finish. Then
>> again, I'm not sure about that, all I see in fb.h is "wait for blit
>> idle, optional"
> 
> 
> fb_write() in fbmem.c calls ->fb_sync() after ->fb_write(), and I've set
> the fb_sync() for each of the drivers, so the behaviour should be
> unchanged for writes.
> 
> The fb_sync() function is also called by fb_read() and 
> fb_get_buffer_offset() (if FB_PIXMAP_SYNC flag is set). I don't know if
> that will adversely affect behaviour.

Well, by just looking at the function names the drivers' fb_syncs call,
it sounds to me that with your patch fb_sync will update the LCD, i.e.
send data to it. Doing that in fb_read sounds totally wrong.

> Note that I haven't actually tested this code since I don't have any of
> the hardware. It was just something I noticed while digging through
> framebuffer driver code.

Ok. Well, I think it's safer to drop these, then, if it's not 100% clear
that there are no unwanted side effects.

 Tomi



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