Re: [Linux-fbdev-devel] [PATCH] [#7001] framebuffer: old image of last app would retain for a while after starting new app

[Mike F. <va...@ge...>]

On Friday 16 March 2012 01:05:10 Zhang, Sonic wrote:
> From: Mike Frysinger [mailto:va...@ge...]
>> On Thursday 15 March 2012 23:34:08 Zhang, Sonic wrote:
>>> From: Mike Frysinger [mailto:va...@ge...]
>>>> On Thursday 15 March 2012 05:23:50 Wu, Aaron wrote:
>>>>> Old image of last application would retain for a while when
>>>>> starting a new application, this patch clear the frambuffer before
>>>>> displaying every time the fb is opened.
>>>> 
>>>> i'm not sure the behavior you describe is wrong.  in fact, i'm pretty
>>>> sure it sounds correct.  if an app writes an image to the framebuffer
>>>> and then quits, that image should stay there indefinitely until
>>>> something else opens the framebuffer and draws their own image.
>>> 
>>> Before the second application draws its own image, the image of last
>>> application has already been displayed on the LCD when the second
>>> application opens the FB device(PPI is enabled when it is opened).
>>> This is not a correct behavior.
>> 
>> sure it is.  if the app wants to clear the screen, it can do so.  generally
>> it's going to anyways by drawing an entire frame.  having the frame buffer
>> driver always clear the screen introduces wasted memory overhead as it
>> does the memset(), and user-visible jank as the device transitions from an
>> initial splash screen to the main userspace app.
>> 
>> this is a policy decision that doesn't really belong in kernel space.  and
>> if it did, it should be agreed upon by all frame buffer users and not just
>> changing a few drivers.
> 
> How can the application clean the screen before it opens the FB device?

why does it need to be before open ?  if the kernel does the memset or 
userspace does the memset, the frame still gets cleared.

in looking at the blackfin framebuffer drivers, i'm not sure they're correct.  i 
don't think the PPI/DMA should be shutdown when the last user space client 
closes it.  fb_release is for releasing all resources when tearing down the 
driver, and fb_blank is runtime management (turning off vsync/hsync and 
powering down the screen).
-mike