Re: [PATCH] PCI: vmd: Delay interrupt handling on MTL VMD controller

[Keith Busch <kbusch@kernel.org>]

On Tue, Sep 03, 2024 at 03:07:45PM +0800, Kai-Heng Feng wrote:
> On Tue, Sep 3, 2024 at 12:29 PM Manivannan Sadhasivam
> <manivannan.sadhasivam@linaro.org> wrote:
> >
> > On Tue, Sep 03, 2024 at 10:55:44AM +0800, Kai-Heng Feng wrote:
> > > Meteor Lake VMD has a bug that the IRQ raises before the DMA region is
> > > ready, so the requested IO is considered never completed:
> > > [   97.343423] nvme nvme0: I/O 259 QID 2 timeout, completion polled
> > > [   97.343446] nvme nvme0: I/O 384 QID 3 timeout, completion polled
> > > [   97.343459] nvme nvme0: I/O 320 QID 4 timeout, completion polled
> > > [   97.343470] nvme nvme0: I/O 707 QID 5 timeout, completion polled
> > >
> > > The is documented as erratum MTL016 [0]. The suggested workaround is to
> > > "The VMD MSI interrupt-handler should initially perform a dummy register
> > > read to the MSI initiator device prior to any writes to ensure proper
> > > PCIe ordering." which essentially is adding a delay before the interrupt
> > > handling.
> > >
> >
> > Why can't you add a dummy register read instead? Adding a delay for PCIe
> > ordering is not going to work always.
> 
> This can be done too. But it can take longer than 4us delay, so I'd
> like to keep it a bit faster here.

An added delay is just a side effect of the read. The read flushes
pending device-to-host writes, which is most likely what the errata
really requires. I think Mani is right, you need to pay that register
read penalty to truly fix this.

> > > +     /* Erratum MTL016 */
> > > +     VMD_FEAT_INTERRUPT_QUIRK        = (1 << 6),
> > >  };
> > >
> > >  #define VMD_BIOS_PM_QUIRK_LTR        0x1003  /* 3145728 ns */
> > > @@ -90,6 +94,8 @@ static DEFINE_IDA(vmd_instance_ida);
> > >   */
> > >  static DEFINE_RAW_SPINLOCK(list_lock);
> > >
> > > +static bool interrupt_delay;
> > > +
> > >  /**
> > >   * struct vmd_irq - private data to map driver IRQ to the VMD shared vector
> > >   * @node:    list item for parent traversal.
> > > @@ -105,6 +111,7 @@ struct vmd_irq {
> > >       struct vmd_irq_list     *irq;
> > >       bool                    enabled;
> > >       unsigned int            virq;
> > > +     bool                    delay_irq;
> >
> > This is unused. Perhaps you wanted to use this instead of interrupt_delay?
> 
> This is leftover, will scratch this.

Maybe you should actually use it instead of making a global? The quirk
says it is device specific, so no need to punish every device if it
doesn't need it (unlikely as it is to see such a thing).