Re: Reading the EDID block for x86 machines

[Antonino Daplas <adaplas@pol.net>]

On Tue, 2003-03-11 at 23:49, James Simmons wrote:
> 
> > I was wondering how to get the EDID block, and my first thought was to
> > implement DDC.  But it is tricky to implement, so why not just get the
> > EDID using an int 0x10 BIOS call?
> 
> DDC is tricky but the nice thing is it is very portable. We can do the 
> BIOS call but that limits it support to intel family of processors.
> I still like to do the BIOS call still because it allows us quick supprot 
> of EDID on intel platforms. 
> 

That's true.  The downside of course is that each driver/hardware has to
have it's own i2c support.  So this may not be very lightweight on first
glance.

Also, the int10 method is simple enough to implement and should
theoretically support all x86 graphics card with a reasonably unbroken
BIOS.  If ppc can get the EDID via OpenFirmware, the x86 can get it via
VBE int10.  Theoretically, other archs that follow some kind of video
standard will have this kind of support also.

The downside of this method is it can only get the EDID block of the
video controller/s initialized at boot time. 
  
> > The EDID block can be read before the kernel goes into protected mode
> > (just a short assembly code in arch/i386/boot/video.S) and this can be
> > saved later on as struct edid_info during the kernel's setup routine.  
> > 
> > One catch though is that that the "empty_zero_page" is becoming too
> > tight, so it will just use the same offset used by the EDD (BIOS
> > Enhanced Disk Drive).  So EDID reading will be disabled if EDD is
> > enabled under the "Processor Type and Features"  option.
> 
> I believe it is possible to read the block after we are in protect mode.
> Look at vesafb and also the aty128 driver allows you to probe the BIOS 
> after we are in protect mode. I suggest we add a feild to struct 
> screeninfo to store the address to the edid pointer. 
>  

Not in this case.  I believe there are very few int10 functions that
have a protected mode interface counterpart.  One of the reasons vesafb
cannot switch modes is because of this.
 
Also the int10 function is a request for the VBE to do DDC, and place
the results to a buffer provided by the caller.  It's possible, of
course, that the VBE has it's own copy of the EDID block, but I don't
know how to search for that. 

> > One of the goals is to build a video data structure that also includes a
> > database of video mode in fb_videomode format.  This can be dynamically
> > created per device using the supported VESA modes, manufacturer's mode
> > and detailed timings mode information in the EDID block. The EDID block
> > also contain information on GTF capability.  With these types of
> > displays, a video mode mode database may not be needed since all
> > possible supported modelines can be calculated.
> > 
> > The advantage of the above is that the driver can switch to different
> > modes without relying on user input (of course, user-entered modelines
> > are still preferred).
> 
> Wow. I'm glad we both think alike. I was thinking the same thing except my 
> hands are tied with working on the upper layer stuff. 
> 
> > Anyway, I need help in testing the code (especially the Detailed
> > Information part).  Just apply the attached patch, disable Enhanced Disk
> > Driver support (under Processor Type and Features), and enable EDID
> > support (under Graphics option).  Finally, add the following line to the
> > fbdev driver that you use (anywhere, but a good place is the init code):
> 
> Diable Enhanced Disk support :-( I have a few suggestions. One we add a 
> pointer to struct screen_info that points to the edid information. We can 
> use phys_to_virt to translate that address into virtual memory. I can 
> create a patch today and store it in the BK tree. 
> 

You can't.  The EDID block was placed in the "empty_zero_page", the same
page shared by screen_info (and the EDD, apm, etc) at boottime .  This
page will get overwritten later on so an explicit save of the contents
must be done.  And because it's only a page (4086 bytes), there may not
be enough room to squeeze in 128 bytes.  The EDD already requires a
buffer at offset 0x600, and I'm not sure how large is the block.  Of
course, if someone tells me where to squeeze in 128 bytes without
overlapping the others, that is infinitely much better.

Tony



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