Am Fri, 20 Mar 2015 16:24:29 +0100
schrieb Geert Uytterhoeven [off-list ref]:

On Fri, Mar 20, 2015 at 3:47 PM, Maxime Ripard
[off-list ref] wrote:

quoted

On Fri, Mar 20, 2015 at 01:37:50PM +0200, Tomi Valkeinen wrote:

quoted

On 15/03/15 00:02, Geert Uytterhoeven wrote:

quoted

On Fri, Mar 13, 2015 at 10:31 PM, Thomas Niederprüm
[off-list ref] wrote:

quoted

Am Tue, 10 Mar 2015 13:28:25 +0200
schrieb Tomi Valkeinen [off-list ref]:

quoted

Also, isn't doing __pa() for the memory returned by vmalloc
plain wrong?

quoted

What was the crash about when using kmalloc? It would be good
to fix defio, as I don't see why it should not work with
kmalloced memory.

The main challenge here is that the memory handed to userspace
upon mmap call needs to be page aligned. The memory returned by
kmalloc has no such alignment, but the pointer presented to the
userspace program gets aligned to next page boundary. It's not
clear to me whether there is an easy way to obtain page aligned
kmalloc memory. Memory allocated by vmalloc on the other hand
is always aligned to page boundaries. This is why I chose to go
for vmalloc.

__get_free_pages()?

I'm not that experienced with mem management, so I have to ask...
__get_free_pages() probably works fine, but isn't vmalloc better
here?

__get_free_pages() will give you possibly a lot more memory than
you need. And the memory is contiguous, so it could be difficult
to allocate a larger memory area. The driver doesn't need
contiguous memory (except in the virtual sense).

vmalloc also returns pages, so the size will be page-aligned. It
doesn't make much of a difference here, since we will only use a
single page in both case (the max resolution of these screens is
128x39, with one bit per pixel).

In that case I recommend get_zeroed_page(), to avoid the vmalloc()
overhead of setting up a mapping.

I looked into get_zeroed_page() too but I thought __get_free_pages()
might be more future proof since get_zeroed_page() will not reserve
enough memory if more than one page is needed. This might occur if a new
controller pops up that has more pixels than bits in a page or the
driver is used on a system with a small page size. Also
get_zeroed_page() is also just calling __get_free_pages() with the
order parameter set to 0. Therefore I think a call like

__get_free_pages(GFP_KERNEL | __GFP_ZERO, get_order(size)) 

does the same as get_zeroed_page() if only one page is needed but has
the ability to reserve more pages if needed.

Thomas