linux-kernel - Re: [PATCH] PCI: designware-ep: Move DBI access to init

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Date:   Thu, 21 Jul 2022 10:30:55 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     kishon@...com, bhelgaas@...gle.com, robh@...nel.org,
        lorenzo.pieralisi@....com, kw@...ux.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
        Om Prakash Singh <omp@...dia.com>,
        Vidya Sagar <vidyas@...dia.com>
Subject: Re: [PATCH] PCI: designware-ep: Move DBI access to init_complete if
 notifier is used

On Thu, Jul 21, 2022 at 02:08:45PM +0530, Manivannan Sadhasivam wrote:
> On Tue, Jul 19, 2022 at 05:38:14PM -0500, Bjorn Helgaas wrote:
> > On Tue, Jul 19, 2022 at 09:13:58AM +0530, Manivannan Sadhasivam wrote:
> > > On Fri, Jul 15, 2022 at 04:39:58PM -0500, Bjorn Helgaas wrote:
> > > > On Wed, Mar 30, 2022 at 11:35:15AM +0530, Manivannan Sadhasivam wrote:
> > > > > For controllers supporting the CORE_INIT notifier, the resources are
> > > > > supposed to be enabled in the init_complete function. Currently,
> > > > > these controllers are enabling the resources during probe time due to
> > > > > the DBI access happens in dw_pcie_ep_init().
> > > > > 
> > > > > This creates the dependency with the host PCIe controller since the
> > > > > resource enablement like PHY depends on host PCIe to be up. For the
> > > > > standalone endpoint usecase, this would never work. So let's move all DBI
> > > > > access to init_complete function if CORE_INIT notifier is used. For the
> > > > > controllers those doesn't support this notifier, this change is a NO-OP.
> > > > > 
> > > > > Cc: Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>
> > > > > Cc: Om Prakash Singh <omp@...dia.com>
> > > > > Cc: Vidya Sagar <vidyas@...dia.com>
> > > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > > ---
> > > > >  .../pci/controller/dwc/pcie-designware-ep.c   | 138 ++++++++++++------
> > > > >  drivers/pci/controller/dwc/pcie-designware.h  |   1 +
> > > > >  2 files changed, 94 insertions(+), 45 deletions(-)
> > > > 
> > > > Sorry this got missed.  Seems like there are two patches to solve the
> > > > same problem:
> > > > 
> > > >   1) This patch, and
> > > >   2) Vidya's patch (https://lore.kernel.org/linux-pci/20220622040133.31058-1-vidyas@nvidia.com/)
> > > > 
> > > > I don't know much about dwc or this issue, but if these patches are
> > > > functionally equivalent, I think Vidya's is a little more attractive
> > > > because:
> > > > 
> > > >   - It's smaller (49 insertions(+), 39 deletions(-)).
> > > > 
> > > >   - "core_init_notifier" looks like sort of a corner-case feature and
> > > >     Vidya's patch doesn't depend on it so it seems more maintainable.
> > > > 
> > > >   - It's more straightforward to read -- it basically just moves
> > > >     things from dw_pcie_ep_init() to dw_pcie_ep_init_complete(), which
> > > >     is exactly the sort of thing I expect if we're doing something out
> > > >     of order.
> > > 
> > > I agree that Vidya's patch is simple but as per the feedback from
> > > Kishon on my previous patch, I had to add some extra logic to make
> > > sure the move of DBI access doesn't affect the non
> > > core_init_notifier platforms.
> > > 
> > > So with my patch, the logic added is essentailly a NO-OP on those.
> > 
> > Can you include the lore URL for Kishon's feedback?  I can't find it.
> 
> https://patchwork.kernel.org/project/linux-pci/patch/1630473361-27198-3-git-send-email-hayashi.kunihiko@socionext.com/#24633629

Thanks!  (Or the canonical permanent URL:
https://lore.kernel.org/r/576457dd-3e66-a3b9-f51c-ea94bc267fdb@ti.com)

> > If we think moving the DBI access is safe on non-core_init_notifier
> > platforms, I'd like to do it everywhere so they're all the same.  I
> > don't want different behavior just to avoid the risk of theoretical
> > problems that we think should not happen.
> 
> One more issue Kishon pointed out was that, in the patch the
> endpoint controller is configured after pci_epc_create(). So he
> raised a concern that if ecp_ops is invoked before the controller
> gets configured fully, it could result in aborts.
> 
> While the concern may be true for non-core_init_notifier platforms
> (I'm not sure though) but I'm certain not for the core_init_notifier
> ones as the EFP drivers only access EPC ops after
> dw_pcie_ep_init_complete().

I really don't understand what "core_init_notifier" does, but it seems
incidental to this issue.  Is there really a connection?  It sounds
like the only reason to check for it is to limit the scope of the
change, not because DBI access is inherently related to
core_init_notifier.

Maybe Vidya's patch needs some enhancement to cover other paths where
DBI may be accessed before we're ready?

Bjorn