Ian Romanick wrote:Tomas Carnecky wrote:
The dri/drm interface seems to be quite low-level. I heard somewhere that different devices have quite different registers and work in a quite different way. If it is true that it would be better to make a more high-level interface where every driver can do it's stuff as it needs. How much low/high-level should the interface be so you get the best performance out of these many different devices and don't have to make workarounds in the drivers. I have the impression that the newest cards have quite high-level chips interfaces (supporting shaders etc.).
The design priciple of the open-source drivers is that the kernel part acts as nothing more than a conduit to shove bits into the chip.
It's the first time I hear that.
Because of that, the interface is pretty raw and varies from chip to chip. For DMA programmed chips, the user-mode driver works by filling buffers with "stuff" and asking the kernel to kick of a DMA operation with the buffer. That's why so much of the ioctl inferface is device-dependent. This doesn't make for "workarounds in the drivers", it just means that the user-mode is written to work with its own low-level kernel-mode interface.
This reminds me to XFree86. XFree86 requires root rights to run, because it accesses the hardware directly (using /dev/mem) and does other dangerous things.
You must be thinking of UtahGLX. DRI make no such requirement on the user. The reason it doesn't have that requirement is that it has part that lives in the kernel. XFree86 & UtahGLX require root because they are completely in user space.
I think that the device driver should avoid that and expose a nice and safe interface to the userspace.
It does.
On advantage of this is that we don't have to modify the kernel drivers very often. Some of the drivers have had regular updates to the user-mode part, but thier kernel driver hasn't been modified in over a year. For us, that's a *big* advantage.
This is certainly a very unimportant thing (for me). Because all other parts of the kernel change regularly, why should the graphics drivers
not?
Because requiring people to upgrade their kernel and 3D driver in lock-step results in a) No benefit to users b) Support headaches for developers whose time is better spent developing. Not only that, putting more stuff in the kernel part makes it more difficult to work on multiple operating systems. The Linux kernel may change every day, but the *BSD kernels do not.
A device driver is not just a wrapper around the device which gives you access to the registers. Even the core components of your computer have a nice interface (your harddisk controller: open/read/write/close etc).
The device driver should expose an interface, which can be used by all
users[1] in a safe way. With the current design, you can't render into a
off-screen buffer easily. Either you use X or DirectFB. But it's not possible to use it in a easy and safe way[2].
There is a nice interface for using a 3D accelerator. It's called OpenGL. The *entire* driver a disk controller lives inside the kernel. In the case of the 3D accelerator, that is impractical. Part of the *driver* lives in the kernel and part of the *driver* lives in user-space.
Nothing about DRI prevents a developer from choosing a different kernel / user split. Based on the size of their kernel modules, I'm pretty sure that both 3dlabs and ATI made a different choice. However, they support Linux only and they aren't distributed with the kernel source. Both of those factors precipitate a different set of technical and practical issues. Because of that, the open-source drivers have the kernel / user split that they have.
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