linux-kernel - RE: [PATCH 2/3] arm64: defconfig: Select CONFIG_RESET

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Date:   Mon, 29 Jun 2020 12:46:17 +0000
From:   Anson Huang <anson.huang@....com>
To:     Arnd Bergmann <arnd@...db.de>, Richard Zhu <hongxing.zhu@....com>
CC:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Sascha Hauer <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Stefan Agner <stefan.agner@...adex.com>,
        "oleksandr.suvorov@...adex.com" <oleksandr.suvorov@...adex.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Leo Li <leoyang.li@....com>, Vinod Koul <vkoul@...nel.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Olof Johansson <olof@...om.net>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>
Subject: RE: [PATCH 2/3] arm64: defconfig: Select CONFIG_RESET_IMX7 by default

Hi, Arnd

> Subject: Re: [PATCH 2/3] arm64: defconfig: Select CONFIG_RESET_IMX7 by
> default
> 
> On Mon, Jun 29, 2020 at 2:09 PM Anson Huang <anson.huang@....com>
> wrote:
> > >
> > > It sounds like your patch 1/3 is not ready to be merged then.
> > >
> > > Please make sure that loading it later does not break other drivers
> > > that depend on it. Other drivers don't have to be able to deal with
> > > missing dependencies if this one is never loaded or disabled at
> > > compile-time. However before you make it possible to turn this into
> > > a loadable module, anything that depends on it must be able to deal
> > > with the modules getting loaded in a random order first.
> > >
> > I searched all driver which uses this reset driver, looks like ONLY
> > i.MX6 PCIe is using it and it ONLY supports built-in.
> 
> Ok, thanks for researching this.
> 
> > and inside this driver, it does NOT support defer probe etc., since I
> > am NOT sure when this PCIe driver will add module support, so do you
> > think if I can make PCI_IMX6 select RESET_IMX7, then it won't break
> > the PCIe function even
> > RESET_IMX7 is set to =m in defconfig, as when PCI_IMX6 is enabled as
> > =y, RESET_IMX7 will be also =y.
> 
> Yes, I think this can work as a short-term workaround, though ideally the PCIe
> driver would also become a loadable module and also support deferred
> probing.
> 
> Having loadable PCIe drivers has traditionally been problematic in Linux, but
> Rob Herring has recently improved this in the series containing patch
> 0c59c06a7c90 ("PCI: host-generic: Support building as modules"), which was
> also intended to help with Android GKI.
> 
> As i.MX uses the designware PCI core support, this may require some more
> changes in PCIE_DW before the i.MX specific part can be a loadable module,
> but it should no longer require changes to the PCI core code. Turning the driver
> into a loadable module is probably not even that hard, but making it possible
> to unload definitely requires adding a proper .remove callback to properly
> unregister the PCIe host bridge.
> 
> I also see a DECLARE_PCI_FIXUP() and a fault handler hook in the pci-imx6.c
> driver, which probably need to get moved into a separate built-in file with a
> few changes.
> 
> And then I noticed a bug in the driver: it hooks the abort handler from an
> initcall whenever the driver is built into the kernel, regardless of which
> machine it is actually running on!
> 
> Regarding deferrer probing, the PCIe host currently relies on the clk controller,
> the regulator and (on imx7d) the reset driver to be probed first. I think making
> it support deferred probing for all three should be straightforward, most
> importantly this means not printing an error and returning -EPROBE_DEFER
> when not all dependencies are there yet.
> 

Thanks for detailed info about PCI loadable module support, I copy our PCI owner
here to help on the support of i.MX6 PCIe driver to support loadable module. Meanwhile,
in this patch series, I will add the short-term workaround as I described upper, and then remove
this short-term workaround either in the i.MX6 PCI driver's loadable module support patch later
or adding a new patch if necessary.

Thanks,
Anson