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

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Date:	Fri, 20 Mar 2015 21:36:59 +0100
From:	Thomas Niederprüm <niederp@...sik.uni-kl.de>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	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.

Am Fri, 20 Mar 2015 17:25:29 +0200
schrieb Tomi Valkeinen <tomi.valkeinen@...com>:

> On 20/03/15 16:47, Maxime Ripard 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).
> 
> Ok, that's not much, then =).
> 
> In that case __get_free_pages sounds fine. Even if the resolution
> would be slightly higher, we're only talking about a page or two
> extra.
> 
> Usually double-underscore in front of a func means "don't call this".
> I don't know why this one has the underscores.

This irritated me as well. But since it turned out that there is even a
section on "get_free_page and Friends" in LDD3 that talks about
__get_free_pages() without any word of caution, I assumed it's ok to
call this functions.

Thomas
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