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

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Date:	Fri, 20 Mar 2015 17:25:29 +0200
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.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 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.

 Tomi



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