Re: [PATCH 4/8] fbdev: ssd1307fb: Use vmalloc to allocate video memory.

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

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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds