can't access video memory?

can't access video memory?

[Zack Smith]

Hi all,

As an experiment I tried accessing a part of (shared) video memory
on my system that is not being used for display data, and this caused the
kernel to crash. I have 6 megs of space and the display is using only half,
but accessing above the 3 meg mark causes the problem. Accessing
the lower 3 megs works fine.

Does anyone know what might cause this?

Thanks,
Zack Smith



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Re: can't access video memory?

[Antonino A. Daplas]

Zack Smith wrote:
> Hi all,
> 
> As an experiment I tried accessing a part of (shared) video memory
> on my system that is not being used for display data, and this caused the
> kernel to crash. I have 6 megs of space and the display is using only half,
> but accessing above the 3 meg mark causes the problem. Accessing
> the lower 3 megs works fine.
> 
> Does anyone know what might cause this?

Depends on the driver.  It's possible that the driver did not ioremap
the entire graphics aperture (ie ioremapped only the first 3 megs).  Or
the driver set info->fix.smem_len to only 3 MB instead of 6.

Some drivers are now differentiating between remapped size vs actual
physical size (radeonfb).  info->screen_size, if not zero, tells the
amount of the remapped size while info->fix.smem_len, the actual physical
size.

Tony


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Re: can't access video memory?

[Jon Smirl]

On 9/10/05, fbui@comcast.net <fbui@comcast.net> wrote:
> > Zack Smith wrote: 
> > > Does anyone know what might cause this? 

On x86 the kernel is limited to 1GB of virtual address space. So
physical memory plus mapped video needs to total less than 1GB for
best performance.

Systems with 1GB of RAM or more want to minimize mapped VRAM. Anything
that is mapped over the minimum pushes kernel data structures out of
normal access and into highmem access which is significantly slower.
For example mapping a 512MB video card on a 1GB system knocks about
10% off from your system performance since it pushes 512MB of RAM into
highmem access mode.

The drivers aren't smart enough to look for less than 1GB of physical
memory and expand their mapped areas.

None of this applies from user space. There are no problems with
mapping the full card.

-- 
Jon Smirl
jonsmirl@gmail.com


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Re: can't access video memory?

[Antonino A. Daplas]

fbui@comcast.net wrote:
> This is for the vesa driver in 2.6.9. Searching the kernel mailing list
> archive I found mention of this bug with a patch to fix it.
> 
> Thus:
>   http://www.ussg.iu.edu/hypermail/linux/kernel/0410.0/0099.html
> 

This patch is already applied to recent 2.6 kernels.

Tony


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