From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EA562C43387 for ; Tue, 1 Jan 2019 01:40:41 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 429AE21019 for ; Tue, 1 Jan 2019 01:40:41 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 429AE21019 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=telegraphics.com.au Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 43TH1y6qWYzDqFL for ; Tue, 1 Jan 2019 12:40:38 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=telegraphics.com.au Authentication-Results: lists.ozlabs.org; spf=none (mailfrom) smtp.mailfrom=telegraphics.com.au (client-ip=98.124.60.144; helo=kvm5.telegraphics.com.au; envelope-from=fthain@telegraphics.com.au; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=telegraphics.com.au Received: from kvm5.telegraphics.com.au (kvm5.telegraphics.com.au [98.124.60.144]) by lists.ozlabs.org (Postfix) with ESMTP id 43TGzn6D2YzDqDq for ; Tue, 1 Jan 2019 12:38:45 +1100 (AEDT) Received: from localhost (localhost.localdomain [127.0.0.1]) by kvm5.telegraphics.com.au (Postfix) with ESMTP id 0F38329CD9; Mon, 31 Dec 2018 20:38:41 -0500 (EST) Date: Tue, 1 Jan 2019 12:38:38 +1100 (AEDT) From: Finn Thain To: Arnd Bergmann Subject: Re: [PATCH v8 24/25] powerpc: Adopt nvram module for PPC64 In-Reply-To: Message-ID: References: <2fe2b8e6395aeacfafcbde590a50922d4e632189.1545784679.git.fthain@telegraphics.com.au> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Greg Kroah-Hartman , Linux Kernel Mailing List , linux-m68k , Paul Mackerras , linuxppc-dev Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Mon, 31 Dec 2018, Arnd Bergmann wrote: > On Sun, Dec 30, 2018 at 4:29 AM Finn Thain wrote: > > > > On Sat, 29 Dec 2018, Arnd Bergmann wrote: > > > > > On Wed, Dec 26, 2018 at 1:43 AM Finn Thain wrote: > > > > > > > +static ssize_t ppc_nvram_get_size(void) > > > > +{ > > > > + if (ppc_md.nvram_size) > > > > + return ppc_md.nvram_size(); > > > > + return -ENODEV; > > > > +} > > > > > > > +const struct nvram_ops arch_nvram_ops = { > > > > + .read = ppc_nvram_read, > > > > + .write = ppc_nvram_write, > > > > + .get_size = ppc_nvram_get_size, > > > > + .sync = ppc_nvram_sync, > > > > +}; > > > > > > Coming back to this after my comment on the m68k side, I notice that > > > there is now a double indirection through function pointers. Have > > > you considered completely removing the operations from ppc_md > > > instead by having multiple copies of nvram_ops? > > > > > > > I considered a few alternatives. I figured that it was refactoring > > that could be deferred, as it would be confined to arch/powerpc. I was > > more interested in the cross-platform API. > > Fair enough. > > > > With the current method, it does seem odd to have a single > > > per-architecture instance of the exported structure containing > > > function pointers. This doesn't give us the flexibility of having > > > multiple copies in the kernel the way that ppc_md does, but it adds > > > overhead compared to simply exporting the functions directly. > > > > > > > You're right, there is overhead here. > > > > With a bit of auditing, wrappers like the one you quoted (which merely > > checks whether or not a ppc_md method is implemented) could surely be > > avoided. > > > > The arch_nvram_ops methods are supposed to optional (that is, they are > > allowed to be NULL). > > > > We could call exactly the same function pointers though either ppc_md > > or arch_nvram_ops. That would avoid the double indirection. > > I think you can have a 'const' structure in the __ro_after_init section, > so without changing anything else, powerpc could just copy the function > pointers from ppc_md into the arch_nvram_ops at early init time, which > should ideally simplify your implementation as well. > This "early init time" could be hard to pin down... It has to be after ppc_md methods are initialized but before the nvram_ops methods get used (e.g. by the framebuffer console). Seems a bit fragile (?) Your suggestion to completely remove the ppc_md.nvram* methods might be a better way. It just means functions get assigned to nvram_ops pointers instead of ppc_md pointers. The patch is simple enough, but it assumes that arch_nvram_ops is not const. The struct machdep_calls ppc_md is not const, so should we worry about dropping the const for the struct nvram_ops arch_nvram_ops? -- > Arnd >