From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Mon, 7 Jun 2004 01:48:12 -0700 From: Eugene Surovegin To: Marius Groeger Cc: linuxppc-dev@lists.linuxppc.org Subject: Re: [RFC] Simple ioremap cache Message-ID: <20040607084812.GA5808@gate.ebshome.net> References: <20040605002915.GA17603@gate.ebshome.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: Sender: owner-linuxppc-dev@lists.linuxppc.org List-Id: On Mon, Jun 07, 2004 at 09:46:16AM +0200, Marius Groeger wrote: > I'm not sure, however, whether your current patch actually saves _TLB_ > misses. Huh? Let's consider two drivers - serial & emac on 44x. Both of them ioremap different parts of the same phys page, but without the patch these will be _different_ virtual mappings, e.g. if they access their respective I/O registers _two_ TLB entries will be used. Here is real example (see my original e-mail): UART0 ioremaps 0x0000000140000200 and gets 0xfdfea200 as a virtual kernel address EMAC0 ioremaps 0x0000000140000800 and gets 0xd1000800. You'll need _two_ TLB entries for these mappings: 0xfdfea000 -> 0x0000000140000000 0xd1000000 -> 0x0000000140000000 With my patch only _one_ TLB mapping will be required for _all_ 4K-sized ioremaps of 0x0000000140000000: 0xfdfea000 -> 0x0000000140000000 440GP has a 64 entry TLB and believe me, when you are running user mode app, there are a lot of TLB misses, and decreasing TLB pressure (e.g. requiring less TLB entries for driver I/O) _will_ save you some TLB misses. > Have you counted them to prove it? No, it seemed quite obvious to me :) > To do this, I think you also need > to flag a bigger virtual page size to the MMU, eg. program a different > PAGESZ_* value (see include/asm-ppc/mmu.h). If you don't, the MMU has to > manage diffent chunks all the same, they just happen to be virtually > contiguous. I don't follow you here, sorry. Could you give some examples with the real TLB contents for the cases you are describing? > So I think what you're saving right now is just mapping entries (which also > is a valid thing to gain). What do you mean "just mapping entries" ? TLB slots contain these "mapping entries", that's the whole purpose of TLB. Eugene. ** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/