From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas =?UTF-8?B?TmllZGVycHLDvG0=?= Date: Fri, 20 Mar 2015 20:36:59 +0000 Subject: Re: [PATCH 4/8] fbdev: ssd1307fb: Use vmalloc to allocate video memory. Message-Id: <20150320213659.11e7f79d@maestro.intranet> List-Id: References: <1423261694-5939-1-git-send-email-niederp@physik.uni-kl.de> <1423261694-5939-5-git-send-email-niederp@physik.uni-kl.de> <20150207111821.GN2079@lukather> <20150207163541.30047a33@maestro.intranet> <20150212151121.GJ2079@lukather> <20150214152212.1643da7d@maestro.intranet> <54FED559.2030506@ti.com> <20150313223128.2a3a682e@maestro.intranet> <550C068E.1070305@ti.com> <20150320144740.GH4255@lukather> <550C3BE9.7080905@ti.com> In-Reply-To: <550C3BE9.7080905@ti.com> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable To: Tomi Valkeinen Cc: Maxime Ripard , Geert Uytterhoeven , Linux Fbdev development list , Jean-Christophe PLAGNIOL-VILLARD , "linux-kernel@vger.kernel.org" Am Fri, 20 Mar 2015 17:25:29 +0200 schrieb Tomi Valkeinen : > 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 Niederpr=C3=BCm > >>> wrote: > >>>> Am Tue, 10 Mar 2015 13:28:25 +0200 > >>>> schrieb Tomi Valkeinen : > >>>>> 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). > >=20 > > 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). >=20 > Ok, that's not much, then =3D). >=20 > 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. >=20 > 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