Received-SPF: pass (google.com: domain of linux-wireless+bounces-1611-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Message-ID: <a85dbfc3-e327-442a-9aab-5115f86944f7@gmail.com>
Date: Mon, 8 Jan 2024 20:08:33 -0800
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [RFC 0/9] PCI: introduce the concept of power sequencing of PCIe
 devices
Content-Language: en-US
To: Bartosz Golaszewski <brgl@bgdev.pl>, Kalle Valo <kvalo@kernel.org>,
 "David S . Miller" <davem@davemloft.net>, Eric Dumazet
 <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>,
 Paolo Abeni <pabeni@redhat.com>, Rob Herring <robh+dt@kernel.org>,
 Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
 Conor Dooley <conor+dt@kernel.org>, Bjorn Andersson <andersson@kernel.org>,
 Konrad Dybcio <konrad.dybcio@linaro.org>,
 Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>,
 Bjorn Helgaas <bhelgaas@google.com>, Heiko Stuebner <heiko@sntech.de>,
 Jernej Skrabec <jernej.skrabec@gmail.com>,
 Chris Morgan <macromorgan@hotmail.com>,
 Linus Walleij <linus.walleij@linaro.org>,
 Geert Uytterhoeven <geert+renesas@glider.be>, Arnd Bergmann <arnd@arndb.de>,
 Neil Armstrong <neil.armstrong@linaro.org>,
 =?UTF-8?Q?N=C3=ADcolas_F_=2E_R_=2E_A_=2E_Prado?= <nfraprado@collabora.com>,
 Marek Szyprowski <m.szyprowski@samsung.com>, Peng Fan <peng.fan@nxp.com>,
 Robert Richter <rrichter@amd.com>, Dan Williams <dan.j.williams@intel.com>,
 Jonathan Cameron <Jonathan.Cameron@huawei.com>,
 Terry Bowman <terry.bowman@amd.com>,
 Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>,
 =?UTF-8?Q?Ilpo_J=C3=A4rvinen?= <ilpo.jarvinen@linux.intel.com>,
 Huacai Chen <chenhuacai@kernel.org>, Alex Elder <elder@linaro.org>,
 Srini Kandagatla <srinivas.kandagatla@linaro.org>,
 Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
 Jim Quinlan <jim2101024@gmail.com>, james.quinlan@broadcom.com
Cc: linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
 devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
 linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
 linux-pci@vger.kernel.org,
 Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
References: <20240104130123.37115-1-brgl@bgdev.pl>
From: Florian Fainelli <f.fainelli@gmail.com>
In-Reply-To: <20240104130123.37115-1-brgl@bgdev.pl>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

Hello,

On 1/4/2024 5:01 AM, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
> 
> During last year's Linux Plumbers we had several discussions centered
> around the need to power-on PCI devices before they can be detected on
> the bus.
> 
> The consensus during the conference was that we need to introduce a
> class of "PCI slot drivers" that would handle the power-sequencing.
> 
> After some additional brain-storming with Manivannan and the realization
> that the DT maintainers won't like adding any "fake" nodes not
> representing actual devices, we decided to reuse the existing
> infrastructure provided by the PCIe port drivers.
> 
> The general idea is to instantiate platform devices for child nodes of
> the PCIe port DT node. For those nodes for which a power-sequencing
> driver exists, we bind it and let it probe. The driver then triggers a
> rescan of the PCI bus with the aim of detecting the now powered-on
> device. The device will consume the same DT node as the platform,
> power-sequencing device. We use device links to make the latter become
> the parent of the former.
> 
> The main advantage of this approach is not modifying the existing DT in
> any way and especially not adding any "fake" platform devices.

There is prior work in that area which was applied, but eventually reverted:

https://www.spinics.net/lists/linux-pci/msg119136.html

and finally re-applied albeit in a different shape:

https://lore.kernel.org/all/20220716222454.29914-1-jim2101024@gmail.com/

so we might want to think about how to have pcie-brcmstb.c converted 
over your proposed approach. AFAIR there is also pcie-rockchip.c which 
has some rudimentary support for voltage regulators of PCIe end-points.

What does not yet appear in this RFC is support for suspend/resume, 
especially for power states where both the RC and the EP might be losing 
power. There also needs to be some thoughts given to wake-up enabled 
PCIe devices like Wi-Fi which might need to remain powered on to service 
Wake-on-WLAN frames if nothing else.

I sense a potential for a lot of custom power sequencing drivers being 
added and ultimately leading to the decision to create a "generic" one 
which is entirely driven by Device Tree properties...

Thanks for doing this!
-- 
Florian