From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from top.free-electrons.com ([176.31.233.9] helo=mail.free-electrons.com) by merlin.infradead.org with esmtp (Exim 4.80.1 #2 (Red Hat Linux)) id 1WFM7f-00024N-87 for linux-mtd@lists.infradead.org; Mon, 17 Feb 2014 11:12:28 +0000 Date: Mon, 17 Feb 2014 08:11:14 -0300 From: Ezequiel Garcia To: Artem Bityutskiy Subject: Re: [PATCH v6 0/3] ubi: Add block interface Message-ID: <20140217111113.GA21782@localhost> References: <1392581041-8099-1-git-send-email-ezequiel.garcia@free-electrons.com> <1392632392.12215.125.camel@sauron.fi.intel.com> <20140217104830.GA21761@localhost> <1392634420.12215.154.camel@sauron.fi.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1392634420.12215.154.camel@sauron.fi.intel.com> Cc: Thomas Petazzoni , Mike Frysinger , Richard Weinberger , Michael Opdenacker , linux-mtd@lists.infradead.org, Piergiorgio Beruto , Brian Norris , David Woodhouse , Willy Tarreau List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Mon, Feb 17, 2014 at 12:53:40PM +0200, Artem Bityutskiy wrote: > On Mon, 2014-02-17 at 07:48 -0300, Ezequiel Garcia wrote: > > On Mon, Feb 17, 2014 at 12:19:52PM +0200, Artem Bityutskiy wrote: > > > Thanks! > > > > > > On Sun, 2014-02-16 at 17:03 -0300, Ezequiel Garcia wrote: > > > > The main target of this patch is supporting lighter, read-only > > > > filesystem, > > > > by putting a squashfs on top of ubiblock. And since squashfs already > > > > provides > > > > a file cache (which is even configurable), a first uncached > > > > implementation > > > > it's good enough for now. > > > > > > Doesn't Linux provide a fake inode for _every_ block device, and uses > > > page-cache to cache pages? > > > > > > There is even entire special-purpose file-system for managing these fake > > > block device inodes, see fs/block_dev.c > > > > > > static struct file_system_type bd_type = { > > > .name = "bdev", > > > .mount = bd_mount, > > > .kill_sb = kill_anon_super, > > > }; > > > > > > I am really not sure which other cache do we need. The only thing coming > > > to my mind is about emulating a HW disk cache in software. But I would > > > not go that far without a really good reason. > > > > > > > If we ever re-add the write support, it would be natural to implement it > > using a 1-LEB cache. Block sectors are much much smaller than LEB's > > (around three orders of magnitude), so we would want to read an entire > > LEB into a 1-LEB buffer, update the buffer, and then write it back. > > > > In this scheme, it's almost straightforward to keep this buffer around > > as a cache, and only write-back to disk when the new read/write requests > > asks for a different LEB. > > > > IIRC, this is the main reason why we had the 1-LEB cache. > > May be. But you are submitting R/O driver. And yet talk about caches and > write support in several places. Caches are only relevant to write > support. Write support is absent. Let's focus on what is present. > > Feel free to put all the thoughts about a possible write support > implementation to a single big comment in the code. That's fine. I do > not object. I just want to focus on what is present. > Totally agreed. Those comments were just a product of my own inertia. If at all possible, take a look at the website doc and I'll push a new round soon. Thanks for the review, -- Ezequiel García, Free Electrons Embedded Linux, Kernel and Android Engineering http://free-electrons.com