linux-kernel - Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

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Date:   Mon, 31 May 2021 21:30:30 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Luca Ceresoli <luca@...aceresoli.net>,
        Pali Rohár <pali@...nel.org>
CC:     <linux-pci@...r.kernel.org>, <linux-omap@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

Hi,

On 31/05/21 7:24 pm, Luca Ceresoli wrote:
> Hi Pali,
> 
> On 31/05/21 15:32, Pali Rohár wrote:
>> On Monday 31 May 2021 11:05:40 Luca Ceresoli wrote:
>>> The PCIe PERSTn reset pin is active low and should be asserted, then
>>> deasserted.
>>>
>>> The current implementation only drives the pin once in "HIGH" position,
>>> thus presumably it was intended to deassert the pin. This has two problems:
>>>
>>>   1) it assumes the pin was asserted by other means before loading the
>>>      driver
>>>   2) it has the wrong polarity, since "HIGH" means "active", and the pin is
>>>      presumably configured as active low coherently with the PCIe
>>>      convention, thus it is driven physically to 0, keeping the device
>>>      under reset unless the pin is configured as active high.
>>>
>>> Fix both problems by:
>>>
>>>   1) keeping devm_gpiod_get_optional(dev, NULL, GPIOD_OUT_HIGH) as is, but
>>>      assuming the pin is correctly configured as "active low" this now
>>>      becomes a reset assertion
>>>   2) adding gpiod_set_value(reset, 0) after a delay to deassert reset
>>>
>>> Fixes: 78bdcad05ea1 ("PCI: dra7xx: Add support to make GPIO drive PERST# line")
>>> Signed-off-by: Luca Ceresoli <luca@...aceresoli.net>
>>>
>>> ---
>>>
>>> Changes v1 -> v2:
>>>  - No changes to the patch
>>>  - Reword commit message according to suggestions from Bjorn Helgaas (from
>>>    another patchset)
>>>  - Add Fixes: tag
>>> ---
>>>  drivers/pci/controller/dwc/pci-dra7xx.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/pci/controller/dwc/pci-dra7xx.c b/drivers/pci/controller/dwc/pci-dra7xx.c
>>> index cb5d4c245ff6..11f392b7a9a2 100644
>>> --- a/drivers/pci/controller/dwc/pci-dra7xx.c
>>> +++ b/drivers/pci/controller/dwc/pci-dra7xx.c
>>> @@ -801,6 +801,8 @@ static int dra7xx_pcie_probe(struct platform_device *pdev)
>>>  		dev_err(&pdev->dev, "gpio request failed, ret %d\n", ret);
>>>  		goto err_gpio;
>>>  	}
>>> +	usleep_range(1000, 2000);
>>
>> Hello! Just a note that this is again a new code pattern in another
>> driver for different wait value of PCIe Warm Reset timeout. I sent email
>> about these issues:
>> https://lore.kernel.org/linux-pci/20210310110535.zh4pnn4vpmvzwl5q@pali/
>>
>> Luca, how did you choose value 1000-2000 us? Do you have some reference
>> or specification which says that this value needs to be used?
> 
> Sadly I haven't access to the PCIe specification.
> 
> I'd be very happy to know what a correct value should be and update my
> patch.

I had given the timing mentioned in the specification here
https://lore.kernel.org/r/023c9b59-70bb-ed8d-a4c0-76eae726b574@ti.com

The PCI EXPRESS CARD ELECTROMECHANICAL SPECIFICATION defines the Power
Sequencing and Reset Signal Timings in Table 2-4. Please also refer Figure
2-10: Power Up of the CEM.

╔═════════════╤══════════════════════════════════════╤═════╤═════╤═══════╗
║ Symbol      │ Parameter                            │ Min │ Max │ Units ║
╠═════════════╪══════════════════════════════════════╪═════╪═════╪═══════╣
║ T PVPERL    │ Power stable to PERST# inactive      │ 100 │     │ ms    ║
╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
║ T PERST-CLK │ REFCLK stable before PERST# inactive │ 100 │     │ μs    ║
╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
║ T PERST     │ PERST# active time                   │ 100 │     │ μs    ║
╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
║ T FAIL      │ Power level invalid to PERST# active │     │ 500 │ ns    ║
╟─────────────┼──────────────────────────────────────┼─────┼─────┼───────╢
║ T WKRF      │ WAKE# rise – fall time               │     │ 100 │ ns    ║
╚═════════════╧══════════════════════════════════════╧═════╧═════╧═══════╝

The de-assertion of #PERST is w.r.t both power stable and refclk stable.

I'm yet to validate this patch, but IIRC devm_gpiod_get_optional(dev,
NULL, GPIOD_OUT_HIGH) will already de-assert the PERST line. Please note
the board here can have various combinations of NOT gate before the gpio
line is actually connected to the connector.

Thanks
Kishon