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Wed, 6 May 2026 17:48:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778089725; bh=cfE/nyxXJocKO1O7pZ91ACi5Q1vHGPeBdIDXW/fileA=; h=Date:From:To:Cc:Subject:In-Reply-To:From; b=L5WmSJSgBp9CROV2OHCeUtlcnNEjgXbMK0hfQbxPLkKUINNC/rzw1xlJ1PO0ffzNW e7Ge5gg6idpYzv2yCbE5KUtKsQqVyVLhMSEh6swycc/WtPoVTE8o4rKaBhShVuNW/t 97ivXP9yFXg7Z0dFnETVEEcnI7RsRCMNxJWfiU0p43hBnNUJGLBB6uOUJNEY5M844O Qq14sClcFECoNJq0ev5P3miyBRH7rNgnuJZ9tgmUvWQh3aqHEAgfo0w62bxBlA5uge Y/O1SIz0K8x6pDbegDm2H+X0iTQKOC2ewr7TH1zljvhgjB0pFIPyjl213/Wv0bj/Vn DJQ4mmcoNDmxQ== Date: Wed, 6 May 2026 12:48:44 -0500 From: Bjorn Helgaas To: Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= Cc: Bjorn Helgaas , Manivannan Sadhasivam , Lorenzo Pieralisi , Magnus Lindholm , Matt Turner , Richard Henderson , Christophe Leroy , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Dexuan Cui , Krzysztof =?utf-8?Q?Ha=C5=82asa?= , Lukas Wunner , Oliver O'Halloran , Saurabh Singh Sengar , Shuan He , Srivatsa Bhat , Ilpo =?utf-8?B?SsOkcnZpbmVu?= , linux-pci@vger.kernel.org, linux-alpha@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v6 24/24] PCI/sysfs: Limit BAR resize attribute scope to platforms with PCI mmap Message-ID: <20260506174844.GA792825@bhelgaas> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260505231945.GB1410272@rocinante> On Wed, May 06, 2026 at 08:42:00AM +0900, Krzysztof Wilczyński wrote: > > > > The only platform without these aforementioned defines is Alpha, which is > > > > conventional PCI only and cannot have ReBAR. So this guard removes dead > > > > sysfs code on platforms where it can never be executed. > > > > > > Having a closer look: > > > > > > resource_resize_attr_is_visible() > > > pci_rebar_get_current_size() <- returns -ENOTSUPP, so is_visible callback returns 0 > > > pci_rebar_find_pos() > > > pos = pdev->rebar_cap <- set to 0 on a conventional PCI > > > if (!pos) > > > return -ENOTSUPP <- no ReBAR support > > > > > > The pdev->rebar_cap is set during PCI enumeration: > > > > > > pci_init_capabilities() > > > pci_rebar_init() > > > pdev->rebar_cap = pci_find_ext_capability() > > > pci_find_next_ext_capability() > > > if (dev->cfg_size <= PCI_CFG_SPACE_SIZE) > > > return 0; <- dev->cfg_size set to 256 here for conventional PCI > > > > > > The PCI_CFG_SPACE_SIZE is 256 here. > > > > > > When the platform has support for PCI Express, the dev->cfg_size is then > > > set to 4096. On an architecture that supports conventional PCI only (such > > > as Alpha), the pdev->rebar_cap will be set to 0, the is_visible callback > > > will then return 0, and the resize sysfs attribute is never created, as > > > such, the __resource_resize_store() callback will never be executed. > > > > > > To make the connetion here to the #ifdef guards: > > > > > > For the pci_rebar_get_current_size() to return >= 0, the device needs > > > PCI Express and extended configuration space support. As of today, > > > every architecture with PCI Express support defines HAVE_PCI_MMAP or > > > ARCH_GENERIC_PCI_MMAP_RESOURCE. > > > > > > I hope the reasoning here works. > > > > We're talking about this #ifdef: > > > > +#if defined(HAVE_PCI_MMAP) || defined(ARCH_GENERIC_PCI_MMAP_RESOURCE) > > static ssize_t __resource_resize_show(struct device *dev, int n, char *buf) > > ... > > +#endif > > > > I follow the reasoning now but by next week I won't, so I think it > > requires too much background knowledge. Future changes involving > > HAVE_PCI_MMAP or ARCH_GENERIC_PCI_MMAP_RESOURCE could easily break > > this. > > > > IIUC it's basically doing what "#ifdef CONFIG_PCI_REBAR" or even > > "#ifdef CONFIG_PCI_EXPRESS" would do, if we had such a thing. > > We don't sadly have such guards at the moment, so using the guards like > HAVE_PCI_MMAP or ARCH_GENERIC_PCI_MMAP_RESOURCE would be the next best > thing. We already rely on these throughout the pci-sysfs.c already, and > will also rely on either for the static sysfs attributes, so if there was > some issues with either of these, if these "broke" somehow, then we would > have other more severe problems. Note, that architectures rely on these > macros to let us know about what "feature" is expected to be enabled, so to > speak, so I would imagine nobody would break this accidentally (famous last > words) and perhaps with extreme caution would do it on purpose. > > > How terrible would it be if we just accepted this dead code on Alpha? > > I would prefer not to build any dead code anywhere if it can be helped, > especially with as little as two lines of code added. Plus, most of the > code setting up resources that are not specific to Alpha, would have been > disabled using exactly the same guard as the one proposed to be added > here, not like this sets any precedent or adds something new. OK, let's leave it as-is. It's confusing to read, but no more so than other existing uses.