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    SubjectRe: [RFC][PATCH 5/10] PCI PM: Avoid touching devices behind bridges in unknown state (rev. 2)
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    On Monday 05 January 2009, Pavel Machek wrote:
    > Hi!
    >
    > > Subject: PCI PM: Avoid touching devices behind bridges in unknown state (rev. 2)
    > > From: Rafael J. Wysocki <rjw@sisk.pl>
    > >
    > > It generally is better to avoid accessing devices behind bridges that
    > > may not be in the D0 power state, because in that case the bridges'
    > > secondary buses may not be accessible. For this reason, during the
    > > early phase of resume (ie. with interrupts disabled), before
    > > restoring the standard config registers of a device, check the power
    > > state of the bridge the device is behind and postpone the restoration
    > > of the device's config space, as well as any other operations that
    > > would involve accessing the device, if that state is not D0.
    >
    > I'm not sure if this is good idea.
    >
    > Either pci config space needs to be restored early, or it can wait.
    >
    > Sometimes restoring it early and sometimes restoring it late seems
    > harmful: it will make code harder to understand and harder to test.

    Unfortunately, we need to restore it early at least for some devices (bridges
    and PCI Express ports) and I don't think it is generally safe to go and restore
    it early for every device (as explained in this changelog).

    > > In such cases the restoration of the device's config space will be
    > > retried during the "normal" phase of resume (ie. with interrupts
    > > enabled), so that the bridge can be put into D0 before that happens.
    >
    > If drivers have to assume it is restored late, anyway... why not do it
    > late, always?

    Because it breaks.

    Thanks,
    Rafael


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