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Date:   Wed, 23 Jan 2019 15:40:24 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To:     Jianjun Wang <jianjun.wang@mediatek.com>
Cc:     Bjorn Helgaas <helgaas@kernel.org>, ryder.lee@mediatek.com,
        robh+dt@kernel.org, matthias.bgg@gmail.com,
        linux-pci@vger.kernel.org, mark.rutland@arm.com,
        devicetree@vger.kernel.org, youlin.pei@mediatek.com,
        linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org,
        honghui.zhang@mediatek.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/2] PCI: mediatek: Add controller support for MT7629
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On Mon, Dec 24, 2018 at 07:40:28PM +0800, Jianjun Wang wrote:
> On Thu, 2018-12-20 at 12:20 -0600, Bjorn Helgaas wrote:
> > On Tue, Dec 18, 2018 at 05:19:24PM +0800, Jianjun Wang wrote:
> > > 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.
> > > 
> > > 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.
> > 
> > If you literally get 0xffff_ffff_0000_0000 when reading the BAR, that
> > is out of spec because the low-order 4 bits of a 64-bit memory BAR
> > cannot all be zero.
> > 
> > A 64-bit BAR consumes two DWORDS in config space.  For a 64-bit BAR0,
> > the DWORD at 0x10 contains the low-order bits, and the DWORD at 0x14
> > contains the upper 32 bits.  Bits 0-3 of the low-order DWORD (the
> > one at 0x10) are read-only, and in this case should contain the value
> > 0b1100 (0xc).  That means the range is prefetchable (bit 3 == 1) and
> > the BAR is 64 bits (bits 2:1 == 10).
> 
> Sorry, I have confused the HW default value and the read value of BAR
> size. The hardware default value is 0xffff_ffff_0000_000c, it's a 64-bit
> BAR with prefetchable range.
> 
> When we start to decoding the BAR, the read value of BAR0 at 0x10 is
> 0x0c, and the value at 0x14 is 0xffff_ffff, so the read value of BAR
> size is 0xffff_ffff_0000_0000, which will be decoded to 0xffff_ffff, and
> it will be set to the end value of BAR0 resource in the pci_dev.
> > 
> > > The calculated BAR size will be 0 in 32-bit platform since the
> > > phys_addr_t is a 32bit value in 32-bit platform.
> > 
> > Either (1) this is a hardware defect that feeds incorrect data to the
> > BAR size calculation, or (2) there's a problem in the BAR size
> > calculation code.  We need to figure out which one and work around or
> > fix it correctly.
> 
> The BAR size is calculated by the code (res->end - res->start + 1) is
> fine, I think it's a hardware defect because that we can not change the
> hardware default value or just disable it since we don't using it.

Apologies for the delay in getting back to this.

This looks like a kernel defect, not a HW defect.

I need some time to make up my mind on what the right fix for this
but it is most certainly not this patch.

Lorenzo