Message-ID: <2a465222-342a-418b-95af-9948f6ce9065@linux.dev>
Date:   Sat, 1 Oct 2022 17:50:07 -0600
From:   Jonathan Derrick <jonathan.derrick@...ux.dev>
To:     Pali Rohár <pali@...nel.org>,
        Lukas Wunner <lukas@...ner.de>
Cc:     Vidya Sagar <vidyas@...dia.com>, bhelgaas@...gle.com,
        robh+dt@...nel.org, krzysztof.kozlowski+dt@...aro.org,
        lpieralisi@...nel.org, kw@...ux.com, thierry.reding@...il.com,
        jonathanh@...dia.com, mani@...nel.org,
        Sergey.Semin@...kalelectronics.ru, jszhang@...nel.org,
        linux-pci@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        kthota@...dia.com, mmaddireddy@...dia.com, sagar.tv@...il.com,
        Marek Behún <kabel@...nel.org>
Subject: Re: [PATCH V1 0/4] GPIO based PCIe Hot-Plug support



On 10/1/2022 10:20 AM, Pali Rohár wrote:
> On Saturday 01 October 2022 18:00:25 Lukas Wunner wrote:
>> Adding Marek, Pali & Jon to cc as they've worked on somewhat similar
>> functionality:
>>
>> https://lore.kernel.org/linux-pci/20220927141926.8895-1-kabel@kernel.org/
>> https://lore.kernel.org/linux-pci/1581120007-5280-1-git-send-email-jonathan.derrick@intel.com/
>>
>> On Sat, Oct 01, 2022 at 12:57:43AM +0530, Vidya Sagar wrote:
>>> To support the Hot-plug feature, PCIe spec has a well-defined model for
>>> hardware implementation and software programming interface. There are also
>>> some architectures/platforms where the Hot-plug feature is implemented in a
>>> non-standard way and software support for the respective implementations is
>>> available with the kernel. This patch series attempts to add support for one
>>> such non-standard way of supporting the Hot-plug feature where a single GPIO
>>> is used to detect and report the Hot-Plug and Unplug events to the SW.
>>> The platforms that can use this piece of software need to have GPIO routed
>>> from the slot to the controller which can indicate the presence/absence of
>>> the downstream device through its state. This GPIO should also have the
>>> capability to interrupt the system when the connection/disconnection event
>>> takes place.
>>> A GPIO Hot-plug framework is written which looks for a "hotplug-gpios" named
>>> GPIO entry in the corresponding device-tree entry of the controller and
>>> registers a hot-pluggable slot with the Hot-plug framework.
>>> The platform drivers of the PCIe host bridges/root ports can register with the
>>> aforementioned GPIO Hot-Plug framework along with ops to perform any platform
>>> specific tasks during Hot-Plug/Unplug events.
>>>
>>> Oza Pawandeep made an attempt to upstream support for a similar Hot-plug
>>> feature implementation at a platform level, but the implementation as such
>>> was very specific to that platform (at least the way I understood it).
>>> https://patchwork.kernel.org/project/linux-pci/patch/1504155029-24729-2-git-send-email-oza.oza@broadcom.com/
>>> https://patchwork.kernel.org/project/linux-pci/patch/1504155029-24729-3-git-send-email-oza.oza@broadcom.com/
>>> https://patchwork.kernel.org/project/linux-pci/patch/1504155029-24729-4-git-send-email-oza.oza@broadcom.com/
>>> This current series also attempts to address that by extracting out all the
>>> common code to do with GPIO and Hot-plug core framework and expecting the
>>> platform drivers to only register/unregister with the GPIO framework. So,
>>> @Oza, could you try using the GPIO framework from this series and enable
>>> Hot-plug support for your platform if it still makes sense?
> 
> Hello!
> 
> Would not it better to rather synthesise PCIe Slot Capabilities support
> in your PCIe Root Port device (e.g. via pci-bridge-emul.c) and then let
> existing PCI hotplug code to take care for hotplugging? Because it
> already implements all required stuff for re-scanning, registering and
> unregistering PCIe devices for Root Ports with Slot Capabilities. And I
> think that there is no need to have just another (GPIO based)
> implementation of PCI hotplug.
I did that a few years ago (rejected), but can attest to the robustness 
of the pcie hotplug code on non-hotplug slots.
https://lwn.net/Articles/811988/


> 
> Similar thing Marek and me have implemented for PCIe link state events
> in patch series with Lukas pointed.
> 
>>> @Rob,
>>> Regarding the DT documentation change to add about 'hotplug-gpios, I'm not
>>> sure if pci.txt is the right place or the dt-schema repository
>>> i.e https://github.com/devicetree-org/dt-schema
>>> But, in the interest of keeping all the changes related to this feature in the
>>> the same repository, I made the changes to the pci.txt file in this repo itself.
>>> Please let me know if the documentation change needs to be moved to the other
>>> repo.
>>>
>>> The Changes have been tested on the Tegra234 platform.
>>>
>>> Vidya Sagar (4):
>>>    dt-bindings: Add "hotplug-gpios" PCIe property
>>>    PCI/hotplug: Add GPIO PCIe hotplug driver
>>>    PCI: tegra194: Add support to configure a pluggable slot
>>>    PCI: tegra194: Enable GPIO based Hot-Plug support
>>>
>>>   Documentation/devicetree/bindings/pci/pci.txt |   4 +
>>>   drivers/pci/controller/dwc/pcie-tegra194.c    |  85 +++++++-
>>>   drivers/pci/hotplug/Kconfig                   |  11 +
>>>   drivers/pci/hotplug/Makefile                  |   1 +
>>>   drivers/pci/hotplug/gpio_php.c                | 200 ++++++++++++++++++
>>>   drivers/pci/hotplug/gpiophp.h                 |  40 ++++
>>>   6 files changed, 334 insertions(+), 7 deletions(-)
>>>   create mode 100644 drivers/pci/hotplug/gpio_php.c
>>>   create mode 100644 drivers/pci/hotplug/gpiophp.h
>>>
>>> -- 
>>> 2.17.1
>>>

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