[Nouveau] Framebuffer double buffering (via FBIOPAN_DISPLAY)

Sun Jul 5 23:56:38 UTC 2020

Intel (i915) uses the same fbdev emulation layer, so should be largely
identical. Just make sure you're using a v4.12+ kernel -- I believe you
mentioned that your Intel system was using 4.9, so that's expected to not
support overallocation.

You might be interested instead in making a kms client which does all this,
as fbdev is deprecated and is unlikely to see any further development (even
to extend the emulation layer to provide features various dedicated drivers
may have had). Some samples available in e.g. kmscube:
https://cgit.freedesktop.org/mesa/kmscube/ (it used to be much simpler, but
has grown various demo features over the years).

Good luck!

  -ilia

On Sun, Jul 5, 2020 at 7:51 PM Michael T. Kloos <michael at michaelkloos.com>
wrote:

> Oops, the FBIOPAN_DISPLAY ioctl error was a stupid mistake on mistake on
> my part.  Thanks for the info on where the validation code was.  Comparing
> against that made it easy to find the mistake.  It's working now,
> well...where I have been able to get over-allocation working!  Thanks for
> the help.  If I can't get the over-allocation to work on my other systems,
> I'll take it up in Intel Graphics specific channels.  While the code is a
> mess right now, here is a link to the program:
> https://github.com/EchelonX-Ray/ttygraphics .  I'm just trying to learn
> about this and you really helped.
> On 7/5/20 7:17 PM, Ilia Mirkin wrote:
>
> Check fb_pan_display in drivers/video/fbdev/core/fbmem.c for the
> argument validation of the FBIOPAN_DISPLAY ioctl.
>
> CONFIG_FB_NVIDIA is a dedicated fbdev driver, not compatible with
> nouveau (it takes over the relevant PCI device). It also won't support
> your Pascal GPU, I believe -- I think it MIGHT support the Tesla
> generation (i.e. G80..GT21x) but definitely not anything newer. Most
> likely it supports pre-G80 GPUs, and again, not compatible with
> nouveau.
>
> DRM-based drivers use the DRM <-> FBDEV emulation layer to provide an
> fbdev interface for legacy applications. The fbdev emulation acts as
> sort of a regular DRM client and exposes the FBDEV API.
>
> The overallocation logic was added to this emulation layer in commit
> 5f15257689bfa, which is included in v4.12 and later kernels.
>
> Hope this helps,
>
>   -ilia
>
> On Sun, Jul 5, 2020 at 7:02 PM Michael T. Kloos<michael at michaelkloos.com> <michael at michaelkloos.com> wrote:
>
> Well...it's been a bit of a mixed bag.  Setting
> drm_kms_helper.drm_fbdev_overalloc=200 set the vinfo.yres_virtual to
> 2160 as opposed to 1080 (My monitor vertical resolution)  This fixed the
> mmap() problem.  However, it only worked on my main workstation.  My
> laptop running Intel graphics wasn't affected by the change in kernel
> cmdline.  My workstation is a custom build from a few years ago with a
> GTX 1060 6GB, running Gentoo(Kernel version: 5.4) machine, and my laptop
> running Manjaro.  I also can not get the IOCTL FBIOPAN_DISPLAY to work.
> It is now giving me an invalid argument error.  I have set vinfo.xoffset
> and vinfo.yoffset to 0 earlier in my program to try to eliminate them as
> variables.
>
> Other factors that may be at play:
>
> -I am trying to run this from an alternate TTY.  I have a graphical
> environment on TTY7 with x.org.  I think it is using the DRM interface.
> Maybe there is a conflict.
> -Maybe I am missing a driver in my Gentoo kernel config?  I'm still
> learning about it, but going into my workstation's kernel config, I
> noticed that CONFIG_FB_NVIDIA was set to "n".
>
> Do you know when CONFIG_DRM_FBDEV_OVERALLOC went into the kernel?  I was
> about to try this on my MacBook Pro running Gentoo, but I will have to
> recompile upgrade the kernel before I can give a report (It is currently
> running: 4.9).  I couldn't find it in menuconfig so I doubt the cmdline
> would make a difference.
>
> I really appreciate the help.  I've been looking header files and a such
> trying to cobble together some information to figure this out.  I hate
> to bother you with this because it seems slightly off topic.
>
> On 7/5/20 3:45 PM, Ilia Mirkin wrote:
>
> Try booting with drm_kms_helper.drm_fbdev_overalloc=200 and see if it
> works with that. (There's also CONFIG_DRM_FBDEV_OVERALLOC which sets
> the default.)
>
> Cheers,
>
>    -ilia
>
> On Sun, Jul 5, 2020 at 3:41 PM <michael at michaelkloos.com> <michael at michaelkloos.com> wrote:
>
> I am not familiar with that setting, but I have really struggled to find documentation on dealing with the framebuffer.  Referring to this guide, "http://betteros.org/tut/graphics1.php#doublebuffer" <http://betteros.org/tut/graphics1.php#doublebuffer>, I attempted to set the mmap allocation size to double, but it caused the mmap to fail.  I no longer believe that it is a driver issue, though, because I just tried it again on a dell laptop with Intel HD Graphics 4400 to the same failure.
>
> On Jul 5, 2020 12:35, Ilia Mirkin <imirkin at alum.mit.edu> <imirkin at alum.mit.edu> wrote:
>
> Are you setting the overallocation to 200?
>
> On Sun, Jul 5, 2020 at 3:41 AM Michael T. Kloos<michael at michaelkloos.com> <michael at michaelkloos.com> wrote:
>
> Does NOUVEAU support mmaping a double-sized Framebuffer?
> When attempting to run, where fd refers to "/dev/fb0":
>
> mmap(ptr, screensize * 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
>
> I get back an invalid argument error.  This doesn't happen if I only
> request a single screensize.  Is this a limitation of the driver?
>
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> --
> *Michael Thomas Kloos*
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> FAA Certified Sport Pilot (ASEL)
> Personal Website: http://www.michaelkloos.com/
>
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