Re: [PATCH] efifb: allow user to disable write combined mapping.

On 19 July 2017 at 08:22, Linus Torvalds [off-list ref] wrote:

On Tue, Jul 18, 2017 at 2:21 PM, Dave Airlie [off-list ref] wrote:

quoted

Oh and just FYI, the machine I've tested this on has an mgag200 server
graphics card backing the framebuffer, but with just efifb loaded.

Yeah, it looks like it needs special hardware - and particularly the
kind of garbage hardware that people only have on servers.

Why do server people continually do absolute sh*t hardware? It's crap,
crap, crap across the board outside the CPU. Nasty and bad hacky stuff
that nobody else would touch with a ten-foot pole, and the "serious
enterprise" people lap it up like it was ambrosia.

It's not just "graphics is bad anyway since we don't care". It's all
the things they ostensibly _do_ care about too, like the disk and the
fabric infrastructure. Buggy nasty crud.

I've tried to reproduce now on:
Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
using some address space from
02:00.0 3D controller: NVIDIA Corporation GM108M [GeForce 940MX] (rev a2)

And I don't see the issue.

I'll try and track down some more efi compatible mga or other wierd server chips
stuff if I can.

Anyway, rant over. I wonder if we could show this without special
hardware by just mapping some region that doesn't even have hardware
in it as WC. Do we even expose the PAT settings to user space, though,
or do we always have to have some fake module to create the PAT stuff?

I do wonder wtf the hw could be doing that would cause this, but I've no idea
how to tell what difference a write combined PCI transaction would have on the
bus side of things, and what the device could generate that would cause such
a horrible slowdown.

Dave.