Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour

From: Luca Ceresoli <luca@lucaceresoli.net>
To: "Pali Rohár" <pali@kernel.org>
Cc: linux-pci@vger.kernel.org, linux-omap@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	Kishon Vijay Abraham I <kishon@ti.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Rob Herring <robh@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH v2] PCI: dra7xx: Fix reset behaviour
Date: Mon, 31 May 2021 15:54:05 +0200	[thread overview]
Message-ID: <8ff1c54f-bb29-1e40-8342-905e34361e1c@lucaceresoli.net> (raw)
In-Reply-To: <20210531133211.llyiq3jcfy25tmz4@pali>

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@lucaceresoli.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.

-- 
Luca