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

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Message-ID: <CAMuHMdXA87LOYxTwf5G3EZ58dPQJfdgv=3esSJQ_MQqCQciKKw@mail.gmail.com>
Date:	Fri, 20 Mar 2015 16:24:29 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Thomas Niederprüm <niederp@...sik.uni-kl.de>,
	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 3:47 PM, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> On Fri, Mar 20, 2015 at 01:37:50PM +0200, Tomi Valkeinen 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).
>
> 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@...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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