Message-ID: <20220531184638.5ksxaz2ewtc2iq73@mobilestation>
Date:   Tue, 31 May 2022 21:46:38 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        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>,
        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 31, 2022 at 11:09:07AM -0500, Rob Herring wrote:
> On Fri, May 27, 2022 at 08:39:53PM +0300, Serge Semin wrote:
> > 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.
> 

> That is the wrong direction IMO. The idea is that well behaved cases 
> just use the generic code and avoid any driver specific code. The DWC 
> common code is not generic code. It's also keeping with the "don't 
> create mid layers" philosophy.

Got it. Thanks for clarification. So far I has been sure that re-using
the locally implemented specifics was more preferable. It was so
obvious for me that I missed there can be the PCI common code requirements.
Though it would be nice to have it described somewhere in the kernel
docs.

> 
> We have generic 32-bit only accessors too (even though that's broken 
> h/w, it's broken so often we needed generic accessors), so if that's 
> your restriction, then use those. That way, it is very clear which 
> drivers (all of them, not just DWC) use generic accessors, have 
> alignment restrictions, or something completely custom.

Oh, I didn't know about them. Thanks for pointing out on those
methods. I'll use them in my driver then.

> 
> > > 2) Drop the Kirin, Exynos, Histb own config-space re-definition.
> 

> Those drivers are special. They get to keep their special code.

It seems to me my driver will be another special case. But instead of
re-implementing the pci_ops.{read,write} accessors it will use the
dword-aligned generic config read/write functions.

-Sergey

> 
> > > 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).

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