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

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Message-ID: <CAMuHMdVZkDUoRAFw1W+Q4jgVODKRztkqVTHzHF8qONcL_iiNzg@mail.gmail.com>
Date:	Fri, 20 Mar 2015 13:12:21 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	Thomas Niederprüm <niederp@...sik.uni-kl.de>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] fbdev: ssd1307fb: Use vmalloc to allocate video memory.

On Fri, Mar 20, 2015 at 12:37 PM, Tomi Valkeinen <tomi.valkeinen@...com> wrote:
> On 15/03/15 00:02, Geert Uytterhoeven wrote:
>> On Fri, Mar 13, 2015 at 10:31 PM, Thomas Niederprüm
>> <niederp@...sik.uni-kl.de> wrote:
>>> Am Tue, 10 Mar 2015 13:28:25 +0200
>>> schrieb Tomi Valkeinen <tomi.valkeinen@...com>:
>>>> Also, isn't doing __pa() for the memory returned by vmalloc plain
>>>> wrong?
>>>
>>>> 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).

I was responding to the question about obtaining page aligned kmalloc
memory.
If it doesn't need to be physically contiguous, vmalloc() is fine.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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
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