Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:4 as permitted sender) client-ip=2620:137:e000::3:4;
Date:   Tue, 3 Oct 2023 22:07:39 +0200
From:   Lukas Wunner <lukas@wunner.de>
To:     Mario Limonciello <mario.limonciello@amd.com>
Cc:     Mika Westerberg <mika.westerberg@linux.intel.com>,
        bhelgaas@google.com, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org
Subject: Re: [PATCH] PCI: Make d3cold_allowed sysfs attribute read only
Message-ID: <20231003200739.GC16417@wunner.de>
References: <20231002181025.82746-1-mario.limonciello@amd.com>
 <20231003094407.GG3208943@black.fi.intel.com>
 <6291393a-b830-4a37-ad2a-1e23b6ec778e@amd.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <6291393a-b830-4a37-ad2a-1e23b6ec778e@amd.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

On Tue, Oct 03, 2023 at 11:30:54AM -0500, Mario Limonciello wrote:
> On 10/3/2023 04:44, Mika Westerberg wrote:
> > On Mon, Oct 02, 2023 at 01:10:25PM -0500, Mario Limonciello wrote:
> > > Before d3cold was stable userspace was allowed to influence the kernel's
> > > decision of whether to enable d3cold for a device by a sysfs file
> > > `d3cold_allowed`.  This potentially allows userspace to break the suspend
> > > for the system.
> > > 
> > > For debugging purposes `pci_port_pm=` can be used to control whether
> > > a PCI port will go into D3cold and runtime PM can be turned off by
> > > sysfs on PCI end points.
> > > 
> > > Change the sysfs attribute to read-only and simplify the internal kernel
> > > logic to avoid needing to store the userspace request.
> > 
> > I wonder if this ends up breaking some userspace apps? Not objecting
> > though, just wanted to mention ;-)
> > 
> > One thing we could do is to taint the kernel or log a warning if
> > userspace touches this but allow it to do so.
> 
> If we collectively decide it needs to stay RW for userspace compatbility
> then I'll respin it to keep the store call but ignore whatever is written,
> return -EINVAL and emit a pr_warn_once() that it's a no-op.

I'd suggest to return 0 to avoid even the faintest ABI change whiff.

Just emit a warning once explaining that the feature is deprecated and
mark the attribute as such in ABI documentation.

Thanks,

Lukas