linux-kernel - Re: [PATCH] PCI: dwc: Warn only for non-prefetchable memory resource size >4GB

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Message-ID: <20200519182029.GA23677@e121166-lin.cambridge.arm.com>
Date:   Tue, 19 May 2020 19:20:29 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Vidya Sagar <vidyas@...dia.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, jingoohan1@...il.com,
        gustavo.pimentel@...opsys.com,
        Andrew Murray <amurray@...goodpenguin.co.uk>,
        bhelgaas@...gle.com, thierry.reding@...il.com,
        jonathanh@...dia.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, kthota@...dia.com,
        mmaddireddy@...dia.com, sagar.tv@...il.com,
        Alan Mikhak <alan.mikhak@...ive.com>
Subject: Re: [PATCH] PCI: dwc: Warn only for non-prefetchable memory resource
 size >4GB

On Tue, May 19, 2020 at 10:38:39PM +0530, Vidya Sagar wrote:

[...]

> > > > > > diff --git a/drivers/pci/controller/dwc/pcie-designware-host.c b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > index 42fbfe2a1b8f..a29396529ea4 100644
> > > > > > --- a/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > +++ b/drivers/pci/controller/dwc/pcie-designware-host.c
> > > > > > @@ -366,7 +366,8 @@ int dw_pcie_host_init(struct pcie_port *pp)
> > > > > >                       pp->mem = win->res;
> > > > > >                       pp->mem->name = "MEM";
> > > > > >                       mem_size = resource_size(pp->mem);
> > > > > > -                   if (upper_32_bits(mem_size))
> > > > > > +                   if (upper_32_bits(mem_size) &&
> > > > > > +                       !(win->res->flags & IORESOURCE_PREFETCH))
> > > > > >                               dev_warn(dev, "MEM resource size exceeds max for 32 bits\n");
> > > > > >                       pp->mem_size = mem_size;
> > > > > >                       pp->mem_bus_addr = pp->mem->start - win->offset;
> > > > 
> > > > That warning was added for a reason - why should not we log legitimate
> > > > warnings ? AFAIU having resources larger than 4GB can lead to undefined
> > > > behaviour given the current ATU programming API.
> > > Yeah. I'm all for a warning if the size is larger than 4GB in case of
> > > non-prefetchable window as one of the ATU outbound translation
> > > channels is being used,
> > 
> > Is it true for all DWC host controllers ? Or there may be another
> > exception whereby we would be forced to disable this warning altogether
> > ?I think so. As I see from the code, ATU's
> Region-0 is used for config space translation
> Region-1 is used for non-prefetchable memory translation
> Region-2 is used for I/O translation
> So, there is no region reserved for translating prefetchable memory regions.

I am confused. Code in dw_pcie_setup_rc() programs a memory region
into ATU. What defines that memory as non-prefetchable ?

Code in dw_pcie_host_init() retrieves the bridge windows and
there may be multiple memory regions.

How is it determined which one ends up in pp->mem ? DT parsing order ?

I reiterate the point - I need DWC maintainers to chime in to define
how this must work.

I am not conviced that your patch is safe to apply and I need to
understand how HW works.

> > > but, we are not employing any ATU outbound translation channel for
> > 
> > What does this mean ? "we are not employing any ATU outbound...", is
> > this the tegra driver ? And what guarantees that this warning is not
> > legitimate on DWC host controllers that do use the ATU outbound
> > translation for prefetchable windows ?
> Not Tegra driver but Tegra HW. Tegra HW doesn't need any ATU outbound
> translation for prefetchable (for that matter any 1-to-1 mapping to
> generate memory transactions on the PCIe bus).

See above.

Lorenzo

> The Warning is still valid for both Tegra and other DWC based controllers
> for non-prefetchable memory translation.
> 
> > 
> > Can DWC maintainers chime in and clarify please ?
> > 
> > > prefetchable window and they can be greater than 4GB in size for all
> > > right reasons. So, logging a warning for prefetchable region doesn't
> > > seem correct to me. Please let me know if my understanding is wrong.
> > 
> > I think your patch is wrong and it is applied on top of a patch that
> > is wrong too, so I won't apply yours and it is likely I will revert
> > Alan's because it seems to solve nothing (and warn spuriously).
> > 
> > It is time for people who maintain DWC please to speak up because I
> > don't have the HW details required to make a judgment.
> > 
> > Lorenzo
> > 
> > > - Vidya Sagar
> > > > 
> > > > Alan ? I want to understand what's the best course of action before
> > > > merging these patches.
> > > > 
> > > > Lorenzo
> > > >