Nouveau - Dec 2008 - Some ideas on how to mix userspace fifo's with a kernel memory manager

I was thinking of the following system:

Allocate a fifo for userspace, map the fifo, map the fifo registers into 
userspace.
These allocations are therefore pinned, so something to avoid memory 
fragmentation has to be done.

Userspace library fills up an entire fifo, minus a few essential things 
i will mention later. Userspace does ioctl with all the bo's it'll need 
for the fifo. Kernel pins all these bo's, creates several dma objects 
that covers only a single bo (as a form of memory protection). Kernel 
sends back a list of these object handles + a list of ref_cnt values 
that have to inserted after the bo usage is done. Userspace fills in the 
remaining values and fires the fifo.

Later, under memory pressure for example, the memory manager checks 
which bo's can be unpinned.

So you'll still need an ioctl, but you do avoid a copy into kernel 
space, which might hurt for stuff like hostdata transfers.

As usual, comment is appreciated.

Maarten.

On 12/20/2008 10:08 PM, Ben Skeggs wrote:
>
>
> On Sat, Dec 20, 2008 at 11:58 PM, Maarten Maathuis 
> <madman2003 at gmail.com <mailto:madman2003 at gmail.com>>
wrote:
>
>     I was thinking of the following system:
>
>     Allocate a fifo for userspace, map the fifo, map the fifo
>     registers into
>     userspace.
>     These allocations are therefore pinned, so something to avoid memory
>     fragmentation has to be done.
>
>     Userspace library fills up an entire fifo, minus a few essential
>     things
>     i will mention later. Userspace does ioctl with all the bo's
it'll
>     need
>     for the fifo. 
>
>     Kernel pins all these bo's,
>
>  Feel free to give it a try, there's nothing stopping you.  The main 
> reason being that the kernel has to be able to do things such as emit 
> buffer moves, and zero-fill new buffers.  We have a channel allocated 
> for the DRM, but as I discovered doing all the above on that channel 
> has severe performance issues (synchronisation beween client's channel 
> and kernel's channel).
You make a good point, i hadn't considered this requirement.
>
>     creates several dma objects
>     that covers only a single bo (as a form of memory protection). Kernel
>     sends back a list of these object handles
>
> The first major problem is here.  This has been discussed numerous 
> times already, and as good an idea as it sounds, it's not possible to 
> implement on any of the chipsets we support.  On all the 3D object 
> classes, there's exactly 2 methods that take object handles for 
> textures (DMA_TEXTURE*).  Now, on nv4x you can use up to 16 texture 
> images, which would each have separate bos.  See the problem already?  
> The image units have a bit which allow you to select between 
> DMA_TEXTURE0 and DMA_TEXTURE1 but that's all.
I just realised this (i was actually going to reply about this if you 
hadn't).
>
> If you're concerned about protection there's not a great deal
that's
> feasible on pre-nv5x chipsets.  Protecting clients from accessing 
> memory that's not supposed to be accessible by the GPU is all we can 
> do, stopping channels from accessing each others' memory isn't.  On
> nv5x each channel has its own address space, which gives us much more 
> flexibility.
>
>     + a list of ref_cnt values
>     that have to inserted after the bo usage is done. Userspace fills
>     in the
>     remaining values and fires the fifo.
>
>     Later, under memory pressure for example, the memory manager checks
>     which bo's can be unpinned.
>
>
>
>     So you'll still need an ioctl, but you do avoid a copy into kernel
>     space, which might hurt for stuff like hostdata transfers.
>
> If you notice the comments in my GEM code, I don't intend on doing 
> this forever.  I'm currently in the process of moving interstate and 
> don't have access to the system I've been working on the mm stuff 
> from, hopefully in a couple of weeks I'll be able to get back to it.
Out of curiosity, does the current work for nv50? It's tempting to give 
it a try and fiddle with it, but if there's still a lot to do to even 
get it working then I'm probably wasting my time.

Do you intend to something mmap'ish for command buffers?
>
> Ben.
>
>
>
>     As usual, comment is appreciated.
>
>     Maarten.
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