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

[Serge Semin]

On Fri, May 27, 2022 at 07:05:55PM +0300, Serge Semin wrote:
> On Thu, May 26, 2022 at 04:29:30PM -0500, Rob Herring wrote:
> > 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.
> 
> Right. I should, but I would do that in a more generic way. Please see
> the next comment.
> 
> > 
> > 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.
> 
> Thanks for clarification. I should have investigated the problem more
> thoroughly. Now I see what was the reason of that change.  It was
> indeed wrong to blame the commit c2b0c098fbd1 ("PCI: dwc: Use generic
> config accessors") that something was done incorrectly. After a more
> thorough commit inspection I realized that you just replaced the
> dw_pcie_rd_own_conf() and dw_pcie_wr_own_conf() with the generic
> pci_generic_config_read and pci_generic_config_write() as they had
> been equivalent anyway.  I thought they didn't have the same semantic
> by confusing the dw_pcie_{read,write}() and dw_pcie_{read,write}_dbi()
> methods usage (see the _dbi suffix) in the original own PCI
> config-space accessors. So to speak I'll need to drop the Fixes tag
> with your commit hash from the patch.
> 
> Getting back to the own-bus accessors. DW PCIe RP/EP own-config space
> is accessed over the DBI-bus. If the particular platform is designed
> in a way so the DBI MMIO space access has some non-specific
> peculiarities then that platform implements its own read_dbi/write_dbi
> accessors. In case if these callbacks are defined, the driver must
> use them for all DBI MMIO accesses including for the ones performed
> from the subsystem core in the framework of the host own config-space
> setups. As I mentioned in the patch log currently the only platforms
> with such requirement happen to be Histb, Exynos and Kirin DW PCIe. As
> such we can freely get back the generic dw_pcie_rd_own_conf() and
> dw_pcie_wr_own_conf() methods but use the dw_pcie_{read,write}_dbi()
> methods in there in the same way as it is done in the Histb, Exynos
> and Kirin DW PCIe drivers (see their own PCI config-space accessors
> match). Due to that we can drop the pci_ops redefinition from these
> platforms and just use the own-config space accessors for all such
> platforms as it's suggested in this patch. So this modification can be
> re-qualified to the cleanup one then:
> 1) Create the generic own config-space accessors (more portable as
> the DBI-bus access specifics must be always taken into account) as it
> is suggested in this patch already.
> 2) Drop the Kirin, Exynos, Histb own config-space re-definition.

> 3) Drop the dw_pcie_read_dbi() and dw_pcie_write_dbi() methods exporting.

Alas this can't be implemented. I forgot about the inliners defined in the
pcie-designware.h file. But the rest of the denoted above cleanups still
can be (Kirin under question though).

-Sergey

> 
> What do you think?
> 
> -Sergey
> 
> > 
> > Rob