netdev - Re: [PATCH] mwifiex: Add quirk resetting the PCI bridge on MS Surface devices

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Message-ID: <20211018153529.GA2235731@bhelgaas>
Date:   Mon, 18 Oct 2021 10:35:29 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Jonas Dreßler <verdre@...d.nl>
Cc:     Pali Rohár <pali@...nel.org>,
        Amitkumar Karwar <amitkarwar@...il.com>,
        Ganapathi Bhat <ganapathi017@...il.com>,
        Xinming Hu <huxinming820@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Tsuchiya Yuto <kitakar@...il.com>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        Maximilian Luz <luzmaximilian@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Johannes Berg <johannes@...solutions.net>,
        Brian Norris <briannorris@...omium.org>,
        David Laight <David.Laight@...LAB.COM>,
        Vidya Sagar <vidyas@...dia.com>,
        Victor Ding <victording@...gle.com>
Subject: Re: [PATCH] mwifiex: Add quirk resetting the PCI bridge on MS
 Surface devices

On Thu, Oct 14, 2021 at 12:08:31AM +0200, Jonas Dreßler wrote:
> On 10/12/21 17:39, Bjorn Helgaas wrote:
> > [+cc Vidya, Victor, ASPM L1.2 config issue; beginning of thread:
> > https://lore.kernel.org/all/20211011134238.16551-1-verdre@v0yd.nl/]

> > I wonder if this reset quirk works because pci_reset_function() saves
> > and restores much of config space, but it currently does *not* restore
> > the L1 PM Substates capability, so those T_POWER_ON,
> > Common_Mode_Restore_Time, and LTR_L1.2_THRESHOLD values probably get
> > cleared out by the reset.  We did briefly save/restore it [1], but we
> > had to revert that because of a regression that AFAIK was never
> > resolved [2].  I expect we will eventually save/restore this, so if
> > the quirk depends on it *not* being restored, that would be a problem.
> > 
> > You should be able to test whether this is the critical thing by
> > clearing those registers with setpci instead of doing the reset.  Per
> > spec, they can only be modified when L1.2 is disabled, so you would
> > have to disable it via sysfs (for the endpoint, I think)
> > /sys/.../l1_2_aspm and /sys/.../l1_2_pcipm, do the setpci on the root
> > port, then re-enable L1.2.
> > 
> > [1] https://git.kernel.org/linus/4257f7e008ea
> > [2] https://lore.kernel.org/all/20210127160449.2990506-1-helgaas@kernel.org/
> 
> Hmm, interesting, thanks for those links.
> 
> Are you sure the config values will get lost on the reset? If we only reset
> the port by going into D3hot and back into D0, the device will remain powered
> and won't lose the config space, will it?

I think you're doing a PM reset (transition to D3hot and back to D0).
Linux only does this when PCI_PM_CTRL_NO_SOFT_RESET == 0.  The spec
doesn't actually *require* the device to be reset; it only says the
internal state of the device is undefined after these transitions.