Re: [RFC 2/2] pci-expender-bus:Add pcie-root-port to pxb-pcie under arm.

["Daniel P. Berrangé" <berrange@redhat.com>]

On Sat, Feb 15, 2020 at 08:59:28AM +0000, miaoyubo wrote:
> 
> > -----Original Message-----
> > From: Daniel P. Berrangé [mailto:berrange@redhat.com]
> > Sent: Friday, February 14, 2020 6:25 PM
> > To: miaoyubo <miaoyubo@huawei.com>
> > Cc: peter.maydell@linaro.org; shannon.zhaosl@gmail.com;
> > imammedo@redhat.com; qemu-devel@nongnu.org; Xiexiangyou
> > <xiexiangyou@huawei.com>; mst@redhat.com
> > Subject: Re: [RFC 2/2] pci-expender-bus:Add pcie-root-port to pxb-pcie under
> > arm.
> > 
> > On Fri, Feb 14, 2020 at 07:25:43AM +0000, miaoyubo wrote:
> > >
> > > > -----Original Message-----
> > > > From: Daniel P. Berrangé [mailto:berrange@redhat.com]
> > > > Sent: Thursday, February 13, 2020 9:52 PM
> > > > To: miaoyubo <miaoyubo@huawei.com>
> > > > Cc: peter.maydell@linaro.org; shannon.zhaosl@gmail.com;
> > > > imammedo@redhat.com; qemu-devel@nongnu.org; Xiexiangyou
> > > > <xiexiangyou@huawei.com>; mst@redhat.com
> > > > Subject: Re: [RFC 2/2] pci-expender-bus:Add pcie-root-port to
> > > > pxb-pcie under arm.
> > > >
> > > > On Thu, Feb 13, 2020 at 03:49:52PM +0800, Yubo Miao wrote:
> > > > > From: miaoyubo <miaoyubo@huawei.com>
> > > > >
> > > > > Since devices could not directly plugged into pxb-pcie, under arm,
> > > > > one pcie-root port is plugged into pxb-pcie. Due to the bus for
> > > > > each pxb-pcie is defined as 2 in acpi dsdt tables(one for
> > > > > pxb-pcie, one for pcie-root-port), only one device could be plugged into
> > one pxb-pcie.
> > > >
> > > > What is the cause of this arm specific requirement for pxb-pcie and
> > > > more importantly can be fix it so that we don't need this patch ?
> > > > I think it is highly undesirable to have such a per-arch difference
> > > > in configuration of the pxb-pcie device. It means any mgmt app which
> > > > already supports pxb-pcie will be broken and need to special case arm.
> > > >
> > >
> > > Thanks for your reply, Without this patch, the pxb-pcie is also
> > > useable, however, one extra pcie-root-port or pci-bridge or something
> > > else need to be defined by mgmt. app. This patch will could be abandoned.
> > 
> > That's not really answering my question. IIUC, this pxb-pcie device works fine
> > on x86_64, and I want to know why it doesn't work on arm ?
> > Requiring different setups by the mgmt apps is not at all nice because it will
> > inevitably lead to broken arm setups. x86_64 gets far more testing & usage,
> > developers won't realize arm is different.
> > 
> >
> 
> Thanks for replying. Currently, on x86_64, pxb-pcie devices is presented
> in acpi tables but on arm, It is not, only one main host bridge is
> presented for arm in acpi dsdt tables. That's why pxb-pcie works on
> x86_64 but doesn't work on arm. The patch 1/2 do the work to present
> and allocate resources for pxb-pcie in arm.

Yes, this first patch makes sense

> For x86_64, if one device is going to be plugged into pxb-pcie, one
> extra pcie-root-port or pci-bridge have to be defined and plugged on
> pxb-pcie, then the device is plugged on the pcie-root-port or pci-bridge.

> This patch 2/2 just auto defined one pcie-root-port for arm. If this
> patch abandoned, the usage of pxb-pcie would be the same with x86_64,
> therefore, to keep the same step for x86 and arm, this patch 2/2 could
> be abandonded.

Yes, I think abandoning this patch 2 is best. Applications that know
how to use pxb-pcie on x86_64, will already do the right thing on
arm too, once your first patch is merged.

Regards,
Daniel
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