linux-kernel - Re: [PATCH v2 3/3] dwc: PCI: intel: Intel PCIe RC controller driver

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Message-ID: <CAFBinCBa9G+E+vjmQCGBx=zRG80ad1am_1TrNbAMvqKCQU38gw@mail.gmail.com>
Date:   Wed, 28 Aug 2019 21:36:38 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     "Chuan Hua, Lei" <chuanhua.lei@...ux.intel.com>
Cc:     eswara.kota@...ux.intel.com, andriy.shevchenko@...el.com,
        cheol.yong.kim@...el.com, devicetree@...r.kernel.org,
        gustavo.pimentel@...opsys.com, hch@...radead.org,
        jingoohan1@...il.com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, qi-ming.wu@...el.com, kishon@...com
Subject: Re: [PATCH v2 3/3] dwc: PCI: intel: Intel PCIe RC controller driver

On Wed, Aug 28, 2019 at 5:35 AM Chuan Hua, Lei
<chuanhua.lei@...ux.intel.com> wrote:
[...]
> >>>>>> +static int intel_pcie_ep_rst_init(struct intel_pcie_port *lpp)
> >>>>>> +{
> >>>>>> +    struct device *dev = lpp->pci->dev;
> >>>>>> +    int ret = 0;
> >>>>>> +
> >>>>>> +    lpp->reset_gpio = devm_gpiod_get(dev, "reset", GPIOD_OUT_LOW);
> >>>>>> +    if (IS_ERR(lpp->reset_gpio)) {
> >>>>>> +            ret = PTR_ERR(lpp->reset_gpio);
> >>>>>> +            if (ret != -EPROBE_DEFER)
> >>>>>> +                    dev_err(dev, "failed to request PCIe GPIO: %d\n", ret);
> >>>>>> +            return ret;
> >>>>>> +    }
> >>>>>> +    /* Make initial reset last for 100ms */
> >>>>>> +    msleep(100);
> >>>>> why is there lpp->rst_interval when you hardcode 100ms here?
> >>>> There are different purpose. rst_interval is purely for asserted reset
> >>>> pulse.
> >>>>
> >>>> Here 100ms is to make sure the initial state keeps at least 100ms, then we
> >>>> can reset.
> >>> my interpretation is that it totally depends on the board design or
> >>> the bootloader setup.
> >> Partially, you are right. However, we should not add some dependency
> >> here from
> >> bootloader and board. rst_interval is just to make sure the pulse (low
> >> active or high active)
> >> lasts the specified the time.
> > +Cc Kishon
> >
> > he recently added support for a GPIO reset line to the
> > pcie-cadence-host.c [0] and I believe he's also maintaining
> > pci-keystone.c which are both using a 100uS delay (instead of 100ms).
> > I don't know the PCIe spec so maybe Kishon can comment on the values
> > that should be used according to the spec.
> > if there's then a reason why values other than the ones from the spec
> > are needed then there should be a comment explaining why different
> > values are needed (what problem does it solve).
>
> spec doesn't guide this part. It is a board or SoC specific setting.
> 100us also should work. spec only requirs reset duration should last
> 100ms. The idea is that before reset assert and deassert, make sure the
> default deassert status keeps some time. We take this value from
> hardware suggestion long time back. We can reduce this value to 100us,
> but we need to test on the board.
OK. I don't know how other PCI controller drivers manage this. if the
PCI maintainers are happy with this then I am as well
maybe it's worth changing the comment to indicate that this delay was
suggested by the hardware team (so it's clear that this is not coming
from the PCI spec)

[...]
> >>>>>> +static void __intel_pcie_remove(struct intel_pcie_port *lpp)
> >>>>>> +{
> >>>>>> +    pcie_rc_cfg_wr_mask(lpp, PCI_COMMAND_MEMORY | PCI_COMMAND_MASTER,
> >>>>>> +                        0, PCI_COMMAND);
> >>>>> I expect logic like this to be part of the PCI subsystem in Linux.
> >>>>> why is this needed?
> >>>>>
> >>>>> [...]
> >>>> bind/unbind case we use this. For extreme cases, we use unbind and bind
> >>>> to reset
> >>>> PCI instead of rebooting.
> >>> OK, but this does not seem Intel/Lantiq specific at all
> >>> why isn't this managed by either pcie-designware-host.c or the generic
> >>> PCI/PCIe subsystem in Linux?
> >> I doubt if other RC driver will support bind/unbind. We do have this
> >> requirement due to power management from WiFi devices.
> > pcie-designware-host.c will gain .remove() support in Linux 5.4
> > I don't understand how .remove() and then .probe() again is different
> > from .unbind() followed by a .bind()
> Good. If this is the case, bind/unbind eventually goes to probe/remove,
> so we can remove this.
OK. as far as I understand you need to call dw_pcie_host_deinit from
the .remove() callback (which is missing in this version)
(I'm using drivers/pci/controller/dwc/pcie-tegra194.c as an example,
this driver is in linux-next and thus will appear in Linux 5.4)


Martin