linux-kernel - Re: [PATCH 2/2] PCI: mediatek: Add controller support for MT7629

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Date:   Tue, 18 Dec 2018 17:19:24 +0800
From:   Jianjun Wang <jianjun.wang@...iatek.com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
CC:     Bjorn Helgaas <helgaas@...nel.org>, <ryder.lee@...iatek.com>,
        <robh+dt@...nel.org>, <matthias.bgg@...il.com>,
        <linux-pci@...r.kernel.org>, <mark.rutland@....com>,
        <devicetree@...r.kernel.org>, <youlin.pei@...iatek.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <honghui.zhang@...iatek.com>,
        <linux-arm-kernel@...ts.infradead.org>, <jianjun.wang@...iatek.com>
Subject: Re: [PATCH 2/2] PCI: mediatek: Add controller support for MT7629

On Mon, 2018-12-17 at 15:46 +0000, Lorenzo Pieralisi wrote:
> On Mon, Dec 17, 2018 at 08:32:47AM -0600, Bjorn Helgaas wrote:
> > On Mon, Dec 17, 2018 at 04:19:39PM +0800, Jianjun Wang wrote:
> > > On Thu, 2018-12-13 at 08:55 -0600, Bjorn Helgaas wrote:
> > > > On Thu, Dec 06, 2018 at 09:09:13AM +0800, Jianjun Wang wrote:
> > > > > The read value of BAR0 is 0xffff_ffff, it's size will be calculated as 4GB
> > > > > in arm64 but bogus alignment values at arm32, the pcie device and devices
> > > > > behind this bridge will not be enabled. Fix it's BAR0 resource size to
> > > > > guarantee the pcie devices will be enabled correctly.
> > > > 
> > > > So this is a hardware erratum?  Per spec, a memory BAR has bit 0 hardwired
> > > > to 0, and an IO BAR has bit 1 hardwired to 0.
> > > 
> > > Yes, it only works properly on 64bit platform.
> > 
> > I don't understand.  BARs are supposed to work the same regardless of
> > whether it's a 32- or 64-bit platform.  If this is a workaround for a
> > hardware defect, please just say that explicitly.
> 
> I do not understand this either. First thing to do is to describe the
> problem properly so that we can actually find a solution to it.
> 
> Lorenzo

This BAR0 is a 64-bit memory BAR, the HW default values for this BAR is
0xffff_ffff_0000_0000 and it could not be changed except by config write
operation.

The calculated BAR size will be 0 in 32-bit platform since the
phys_addr_t is a 32bit value in 32-bit platform.

Actually MediaTek's HW does not using this BAR0, just omit it when
assign resource is totally fine.

When assign the resource for each device, software will check the
resource alignment first, and the resource of length zero will be
regarded as a bogus alignment resource, it will be ignored and won't
claim a resource parent for it.

When drivers try to enable the PCIe devices, the software will enable
it's resources, but it will return an error number when found a
unclaimed resource, in that case, the flow of enable devices will be
interrupted and PCIe devices won't work properly.

Thanks.