linux-kernel - Re: [PATCH v3 02/13] PCI: dwc: Don't use generic IO-ops for DBI-space access

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Date:   Thu, 26 May 2022 16:29:30 -0500
From:   Rob Herring <robh@...nel.org>
To:     Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc:     Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Serge Semin <fancer.lancer@...il.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
        Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
        Frank Li <Frank.Li@....com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 02/13] PCI: dwc: Don't use generic IO-ops for
 DBI-space access

On Tue, May 17, 2022 at 03:50:47PM +0300, Serge Semin wrote:
> Commit c2b0c098fbd1 ("PCI: dwc: Use generic config accessors") replaced
> the locally defined DW PCIe host controller config-space accessors with
> the generic methods pci_generic_config_read() and
> pci_generic_config_write(). It was intended that the corresponding
> bus-mapping callback returned a correct virtual address of the passed PCI
> config-space register. The problem of the proposed solution was that it
> didn't take into account the way the host config-space is accessed on the
> DW PCIe. Depending on the DW PCIe IP-core synthesize parameters different
> interfaces can be used to access the host and peripheral config/memory
> spaces. The former one can be accessed via the DBI interface, while the
> later ones is reached via the AHB/AXI application bus. In case if the DW
> PCIe controller is configured to have a dedicated DBI interface, the way
> it is mapped into the IO-memory turns to be platform-specific. For such
> setups the DWC PCIe driver provides a set of the callbacks
> dw_pcie_ops.{read_dbi,write_dbi} so the platforms glue-drivers would be
> able to take into account the DBI bus IO peculiarities. Since
> commit c2b0c098fbd1 ("PCI: dwc: Use generic config accessors") these
> methods haven't been utilized during the generic host initialization
> performed by the PCIe subsystem code.
> 
> I don't really know how come there have been no problems spotted for the
> Histb/Exynos/Kirin PCIe controllers so far, but in our case with dword

Because they implement their own pci_ops for the root bus. You should 
too.

Who is 'our case'? 

> aligned IO requirement the generic config-space accessors can't be
> utilized for the host config-space. Thus in order to make sure the host
> config-space is properly accessed via the DBI bus let's get back the
> dw_pcie_rd_own_conf() and dw_pcie_wr_own_conf() methods. They are going to
> be just wrappers around the already defined
> dw_pcie_read_dbi()/dw_pcie_write_dbi() functions with proper arguments
> conversion. These methods perform the platform-specific config-space IO if
> the DBI accessors are specified, otherwise they call normal MMIO
> operations.

The idea was for DWC to not define its own way to have different 
read/write for root bus vs. child bus as many PCI host bridges need the 
same thing. So the host bridge struct now has 2 pci_ops pointers. And 
the mess of function pointer indirection is gone.

Rob