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

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Date:	Fri, 20 Mar 2015 13:37:50 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Thomas Niederprüm 
	<niederp@...sik.uni-kl.de>
CC:	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 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).

 Tomi



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