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

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Date:	Fri, 20 Mar 2015 15:47:40 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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 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).

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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