Re: [Linux-fbdev-devel] Fwd: Re: [Dri-devel] future of DRI?

[Sven L. <lu...@dp...>]

On Sun, Mar 02, 2003 at 06:39:51AM +0800, Antonino Daplas wrote:
> On Sun, 2003-03-02 at 02:42, Jon Smirl wrote: 
> > --- Linus Torvalds <tor...@tr...> wrote:
> > > From: Linus Torvalds <tor...@tr...>
> > > To: Keith Whitwell <ke...@tu...>
> > > CC: Jon Smirl <jon...@ya...>, Ian Romanick
> > > <id...@us...>,
> > >         "DRI developer's list"
> > > <dri...@li...>
> > > Subject: Re: [Dri-devel] future of DRI?
> > > Date: Sat, 1 Mar 2003 10:15:06 -0800 (PST)
> > > 
> > > 
> > > On Sat, 1 Mar 2003, Keith Whitwell wrote:
> > > > 
> > > > Interesting you mention it.  This is what Brian &
> > > I've done in the Mesa 
> > > > embedded branch -- layered the radeon dri driver
> > > on top of fbdev.  I can also 
> > > > build regular DRI drivers from a minimal tree &
> > > sane set of makefiles.
> > > 
> > > Personally, I'd rather see DRI _underneath_ fbdev
> > > rather than on top of. 
> > > Since fbdev would require at least to know of (and
> > > obey) the DRI locking 
> > > code - and would likely want to use all the same DRI
> > > command execution for 
> > > things like blits etc (this is on the assumption
> > > that 2d and 3d will 
> > > eventually use the same engine, which is probably a
> > > safe assumption).
> > > 
> > > I _assume_ that what you really mean is that you use
> > > fbdev really only to
> > > set up the screen modes and do things like
> > > initialize the graphics
> > > buffers.
> > > 
> > > 		Linus
> > > 
> Yes, this is the sanest way.  In my opinion, this is how fbdev and DRI
> should operate: 

Maybe you should have cross posted this to the DRI mailing list, and
maybe even the xfree86 one ?
> 
> 1. fbdev 
> - provide a means to initialize and change the video state. 
> 
> - provide pointers to graphics/rendering memory, MMIO, DMA/ringbuffers 
> 
> - graphics memory may or may not be available to everyone, but the MMIO
> and command buffers will only be available to DRI 
> 
> - fbdev must not touch any registers besides those required to
> initialize the hardware.  No 2D, no 3D. 
> 
> 2. fbaa 
> 
> - or framebuffer acceleration architecture or whatever you want to call
> it.  This will be equivalent to Xfree86's XAA. It provides a 2D
> abstraction layer for clients residing in kernel space (ie fbcon). It
> will have software versions and optionally accelerated versions.  The
> accelerated version has intimate knowledge of the 2D engine, but instead
> of accessing the hardware directly, it will rely on DRM to pass commands
> to the hardware. 

It would be real neat if we could re-use the same code as XFre86 XAA
drivers, but i don't think this will ever happen. but then we are on the
verge of the X 5.0 discution, so ...

> - in its current form, this will be the fb_imageblit(), fb_copyarea()
> and fb_fillrect(). 
> 
> 3. fbcon 
> 
> - this is the console driver that runs on top of fbaa 
> 
> 4. DRM - will get mmio pointer and command buffers from fbdev and will
> generally retain its original functions (interrupt handling, locking,
> arbitration, DMA interface, the works).  It must also provide an
> interface for fbaa. 
> 
> 5. Userland apps - should only see the graphics memory pointer via
> fbdev.  If they need to access the hardware, they have to go through
> DRM. 
> 
> Advantages:  
> 
> 1. fbdev will be secure.  Without access to the MMIO regions, crashing
> the chipset is unlikely or at least difficult.  Even malicious blit
> commands (blits to/from system memory) will not work.  
> 
> 2. Single point of entry for hardware access (DRI).  You can run
> multiple clients trying to access the hardware simultaneously via DRM.  
> And because of DRM's features, it will take care of command
> verification, arbitration, locking, context switching, etc.  
> 
> 3.  Because DRM will handle both 2D and 3D and is pretty much the only
> one with hardware access, performance might actually increase.  

Notice that the radeon X driver already does this when possible.

I think you would still need to do context swaps and such though.

> Disadvantages: 
> 
> 1. very linux specific.  Xfree86 was designed to run on different
> platforms.  Having one code for linux and another for the rest will be
> difficult for XFree86 developers to accept. 

No problem, XFree86 should provide a wrapper around its xaa code, which
would either use the drm when available or use the mmio/whatever accesses
when no drm is available.

When no fbdev is available, it should use its own initialization code,
and that is it. For 5.0, we should lobby xfree86 that they separate more
the initialization code from the rest of it.

> 2. this will break fb-based apps that require chipset access, ie
> DirectFB. 

Why ? They should use the drm also, and everything would be nice, it
would mean a rewrite though.

> 3. a lot of code are difficult to implement in kernel space, ie
> initialization of secondary cards.  Full video bios access can only be
> done, from a practical standpoint, in user space (the Intel 830, for
> instance, may require this). 

Yes, this may be a problem.

> 4. Not all fbdev drivers has a DRI counterpart.  For these chipsets,
> fbaa still has to access the hardware directly. 

Notice that above you spoke about the drm, you would write a drm driver
for each graphic chipset, that it has or not a DRI above it.

> In linux-2.5, fbcon is already separate from fbdev.  Perhaps in 2.7,
> fbdev can be further reduced to a minimal core, moving the rest of the
> code to fbaa.  Exporting the mmio regions to userland must be
> disallowed. 
> 
> Secondly, a module to access DRM services from within the kernel will be
> needed. 

I personnaly like this proposal, but you really need to comunicate it
beyond the linux-fbdev list. The DRI list is where this discution got
started, the above mail from Linus was forwarded from there, and there
are people doing embeded mesa on top of fbdev there, and xfree86 is just
starting to think about 5.0.

Friendly,

Sven Luther
> 
> Any comments? 
> 
> Tony  
> 
> 
> 
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