[linuxsh-dev] Re[2]: Strange behavior when accessing mmap'd framebuffer

[Masahiro A. <m-...@aa...>]

Hi David, I really, really thank you so much. It seems to work fine now
with what you suggested! RTLinux task wake-up time is now below 70
microseconds. I'm very happy with this.

I also would like to take a look at latest round of patches, but I must
provide workable kernel to another developer soon, so for now I'll go
with this hack. I sure will try new code soon.

Thanks again, it really helped us a lot!

On Wed, 4 Jul 2001 18:09:21 +0100
David Mckay <Dav...@st...> wrote:

> On Jul 4, 10:10pm, m-...@aa... wrote:
> > Subject: [linuxsh-dev] Strange behavior when accessing mmap'd framebuffer
> >
> > Hello all,
> >
> > Today I would like to get comments/suggestions on my problem, if you
> > have. I'll really appreciate any info you would provide.
> >
> > I'm running custom SH4 machine with LCD controller (Epson s1d13706) with
> > monochrome LCD, using it as framebuffer device. It can be accessed as
> > /dev/fb0 now.
> > When I access mmap'd framebuffer memory area from user program (by
> > memcpy, for example), sometimes it takes very long time to complete.
> > Writing (unsigned char)0x00 to the area sometimes took more than 700
> > microseconds. Usually it takes just about 10 microseconds.
> >
> 
> We have seen exactly this behaviour. It is caused by the TLB handler code
> (Pre the latest patches). What happens is the dcache aliasing code kicks in,
> causing all TLB entries to be scanned.
> 
> As a quick hack, what I did was change the code in update_mmu_cache() to
> instead of
> 
> if(pte_shared(pte))
> 
> to if(pte_shared(pte) && pte_cacheable(pte))
> 
> Dcache aliasing can't happen if the memory is uncached.
> 
> Anyway, this provided a big performance increase for me.
> 
> We actually were going to rewrite the whole of the way the dcache aliasing was
> done, but as usual Niibe beat us to it:-) That's another bug chunk of work gone
> from our project plan!
> 
> The latest kernel should fix this problem I think, but I haven't tested it yet,
> for framebuffer access, although it appears stable to me for other purposes.
> 
> Hope this helps!

+-------------------------------------+
| Masahiro Abe, Software Engineer     |
| A&D Co., Ltd. of Tokyo, Japan       |
| mailto:m-...@aa...            |
+-------------------------------------+
|This is my opinion, not my employer's|
+-------------------------------------+