linux-kernel - Re: [PATCH] PCI: j721e: Extend j721e_pcie_ctrl

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Date: Tue, 30 Jan 2024 10:20:37 +0530
From: Siddharth Vadapalli <s-vadapalli@...com>
To: Andrew Davis <afd@...com>
CC: <bhelgaas@...gle.com>, <lpieralisi@...nel.org>, <kw@...ux.com>,
        <robh@...nel.org>, <vigneshr@...com>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
        <srk@...com>, <s-vadapalli@...com>
Subject: Re: [PATCH] PCI: j721e: Extend j721e_pcie_ctrl_init() for non syscon
 nodes

Hello Andrew,

On 29/01/24 20:49, Andrew Davis wrote:
> On 1/29/24 4:49 AM, Siddharth Vadapalli wrote:

..

>>       int ret;
>>   -    syscon = syscon_regmap_lookup_by_phandle(node, "ti,syscon-pcie-ctrl");
>> +    scm_conf = of_parse_phandle(node, "ti,syscon-pcie-ctrl", 0);
>> +    if (!scm_conf) {
>> +        dev_err(dev, "unable to get System Controller node\n");
>> +        return -ENODEV;
>> +    }
>> +
>> +    syscon = device_node_to_regmap(scm_conf);
> 
> Turning the entire "simple-bus" region into a regmap using this
> function is just as broken as having it as a "syscon". The core
> problem we are solving by getting rid of the blanket syscon nodes
> is that it causes multiple mappings of the same register. This
> can cause issues with regmap caching, read–modify–write, etc..
> 
> What you want to do is add a subnode to the simple-bus, have that
> encapsulate just the registers used for PCIe, and have the PCIe
> node point to that. Then this patch isn't needed.
> 
> For an example, see how it's done for DSS[0].

Thank you for reviewing the patch. I will implement it similar to what's done
for DSS as you pointed out. However, what about the existing SoCs which make use
of the "ti,syscon-pcie-ctrl" property? Do you suggest that I add another
device-tree property for pointing to the PCIE_CTRL register within the CTRL_MMR
region, or do you suggest that I reuse the existing "ti,syscon-pcie-ctrl"
property differently in the SoCs like J784S4 where the scm_conf node is a
"simple-bus"?

The "ti,syscon-pcie-ctrl" property as defined in the device-tree bindings has
two elements with the first being the phandle to the scm_conf node and the
second being the offset of the PCIE_CTRL register. The newer implementation you
are suggesting will either require a new property which accepts only one element
namely the phandle to the node within scm_conf corresponding to the PCIE_CTRL
register. Will adding a new property be acceptable?

..

-- 
Regards,
Siddharth.