Re: [PATCH update6] drivers: add LCD support

["Miguel Ojeda" <maxextreme@gmail.com>]

On 11/2/06, Andrew Morton <akpm@osdl.org> wrote:
> On Thu, 2 Nov 2006 14:44:56 +0000
> "Miguel Ojeda" <maxextreme@gmail.com> wrote:
>
> > > Now let's say that some of the kernel frame buffer data are in the
> > > data cache and never be invalidate during this example. The
> >
> > Sorry, I don't understand what do you mean with this sentence.
>
> Some CPU architectures experience what Documentation/cachetlb.txt calls
> "virtual aliasing in the data cache".
>
> If you map the same physical page at virtual address A1 and also at another
> virtual address A2 then writes to address A1 do not necessarily appear
> correctly at address A2.  This because the write to A1 is stuck in the CPU
> cache and the CPU hardware doesn't know that read from A2 is accessing the
> same page.
>
> The solutions to this are:
>
> a) add lots of flushing everywhere (see Documentation/cachetlb.txt, I
>    guess) (this documentation is maddening: it uses the term "flush" for
>    both writeback and for invalidation.  In this context,
>    flush==writeback).
>
> b) If you select the correct virtual addresses for A1 and A2 (ie: ensure
>    that the mmap() handler returns an address which correlates with the
>    page's kernel address) then apparently the aliasing goes away.
>
>    So, for example, if the kernel's view of the page is at 0xd0102000
>    then you make sure that userspace's address for the page is at
>    0xnnn02000 (for some length of nnn).
>
>    I don't know what the mmap handler has to do to arrange for this to
>    happen.
>
> c) add `depends on x86' to your Kconfig ;)

I really like portable stuff :)

> davem and rmk are (amongst others) the guys for this stuff.  afaik it isn't
> documented anywhere.

Whoa, thanks you for the long explanation. May 2.6.18-new vmalloc
related functions help correlating userspace & kernel addresses? I
will try them and come with an answer tomorrow.

Quoting http://lwn.net/Articles/2.6-kernel-api/

"Some functions have been added to make it easy for kernel code to
allocate a buffer with vmalloc() and map it into user space. They are:

    void *vmalloc_user(unsigned long size);
    void *vmalloc_32_user(unsigned long size);
    int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
                            unsigned long pgoff);

The first two functions are a form of vmalloc() which obtain memory
intended to be mapped into user space; among other things, they zero
the entire range to avoid leaking data. vmalloc_32_user() allocates
low memory only. A call to remap_vmalloc_range() will complete the
job; it will refuse, however, to remap memory which has not been
allocated with one of the two functions above."
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