linux-kernel - Re: [PATCH v5 20/20] PCI: dwc: Add Baikal-T1 PCIe controller support

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Message-ID: <98179709-1ece-61ab-d43a-fc38a4fd3f67@arm.com>
Date:   Wed, 31 Aug 2022 09:54:14 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Rob Herring <robh+dt@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Frank Li <Frank.Li@....com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        linux-pci@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, willmcvicker@...gle.com
Subject: Re: [PATCH v5 20/20] PCI: dwc: Add Baikal-T1 PCIe controller support

On 2022-08-31 09:36, Robin Murphy wrote:
> On 2022-08-29 16:28, Lorenzo Pieralisi wrote:
> [...]
>>> +static int bt1_pcie_add_port(struct bt1_pcie *btpci)
>>> +{
>>> +    struct device *dev = &btpci->pdev->dev;
>>> +    int ret;
>>> +
>>> +    /*
>>> +     * DW PCIe Root Port controller is equipped with eDMA capable of
>>> +     * working with the 64-bit memory addresses.
>>> +     */
>>> +    ret = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64));
>>> +    if (ret)
>>> +        return ret;
>>
>> Is this the right place to set the DMA mask for the host controller
>> embedded DMA controller (actually, the dev pointer is the _host_
>> controller device) ?
>>
>> How this is going to play when combined with:
>>
>> https://lore.kernel.org/linux-pci/1e63a581-14ae-b4b5-a5bf-ca8f09c33af6@arm.com
>>
>> It is getting a bit confusing. I believe the code in the link
>> above sets the mask so that through the DMA API we are capable
>> of getting an MSI doorbell virtual address whose physical address
>> can be addressed by the endpoint; this through the DMA API.
>>
>> This patch is setting the DMA mask for a different reason, namely
>> setting the host controller embedded DMA controller addressing
>> capabilities.
>>
>> AFAICS - both approaches set the mask for the same device - now
>> the question is about which one is legitimate and how to handle
>> the other.
> 
> Assuming the dw-edma-pcie driver is the relevant one, that already sets 
> its own masks on its own device, so I also don't see why this is here.

Ah, I just found the patch at [1], which further implies that this is 
indeed completely bogus.

Robin.

[1] 
https://lore.kernel.org/dmaengine/20220822185332.26149-23-Sergey.Semin@baikalelectronics.ru/