Re: newbie: fb specs and indirect VRAM access

On Thu, Aug 21, 2008 at 8:54 AM, Geert Uytterhoeven
[off-list ref] wrote:

On Thu, 21 Aug 2008, Jorge Marques Pelizzoni wrote:

quoted

A few days ago I sent the message below asking about framebuffer device
documentation and explaining my specific situation. Nobody aswered and I
wonder why. If those are not questions for this list, can someone please
redirect me to the right forum?

I had hoped somebody else would find time to respond ;-)

quoted

Jorge Marques Pelizzoni escreveu:

quoted

I am considering implementing a frame buffer device for a mobile platform
(LPC2400 on uCLinux) that, as far as I can tell, seems to lack one. Well,
I am a newbie to all things frame buffer-related (though familiar with
kernel/device module implementation) and would most appreciate it if you
could answer the following questions:

1 - are there documents specifying frame buffer devices and how
applications are expected to handle them (in short, their API)?

linux/Documentation/fb/framebuffer.txt
linux/include/linux/fb.h

quoted

quoted

2 - my CPU cannot address "real" VRAM directly, all control being carried
out by means of a parallel interface. I gathered that an fb user is
expected to write to (virtual) VRAM directly via a pointer provided by the
driver and never notifies the driver of changes. Have you seen such a
set-up before and do you think these requirements can be reconciled? The
only way round this I can currently see is making the driver periodically
check the virtual VRAM for changes and transmit only changed parts. All in
all, a costly operation, as everything must be carried out by the same
CPU. Could whatever is used to implement mmap be called into play (without
whatever is used to implement msync)?

linux/Documentation/fb/deferred_io.txt

One note about this. If you're using LPC2400, I think that's an ARM7.
TDMI so no MMU which I think matches your mention of uclinux.
Unfortunately, defio has a dependency on MMU. There are other
possibilities to achieve the same effect but I'm not familiar with the
2400 and I'm not sure if it can do it.

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