fb_write change in your tree

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

Hi James !

What is this change to fb_write() in your tree ? It looks very
wrong to me. You are removing the ability for the driver to
hook it's own fb_write function, which can be a problem. You
basically assume copy_from_user() can be used straight with
the framebuffer as a destination. This is broken. On ppc, for
example, copy_from_user() may do dcbz's cache instruction on
the destination. However, the fb is mapped uncacheable, that
will result in a lot of exceptions during the copy, which is
plain wrong (or possibly the copy failing completely).

What about drivers that are taking great care _NOT_ to map
the framebuffer (or at least not all of it) to avoid having
to ioremap hundreds of megabytes in kernel space ?

A driver should be able to never fill scree_base provided
that it has filled all the accel hooks and provides it's
own fb_read/write functions (which can be slow as they might
have to do mapping/unmapping of fb portions on demand, but
then, mmap is the preferred way for userland to access the
framebuffer content when that is possible).


Ben.




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