linux-kernel - Re: [PATCH 1/2] PCI: of: Override 64-bit flag for non-prefetchable memory below 4GB

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Message-ID: <20210527162130.GA1401058@bjorn-Precision-5520>
Date:   Thu, 27 May 2021 11:21:30 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Punit Agrawal <punitagrawal@...il.com>
Cc:     linux-rockchip@...ts.infradead.org, linux-pci@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        alexandru.elisei@....com, wqu@...e.com, robin.murphy@....com,
        pgwipeout@...il.com, ardb@...nel.org, briannorris@...omium.org,
        shawn.lin@...k-chips.com, robh+dt@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 1/2] PCI: of: Override 64-bit flag for non-prefetchable
 memory below 4GB

On Fri, May 28, 2021 at 12:05:41AM +0900, Punit Agrawal wrote:
> Some host bridges advertise non-prefetable memory windows that are
> entirely located below 4GB but are marked as 64-bit address memory.
> 
> Since commit 9d57e61bf723 ("of/pci: Add IORESOURCE_MEM_64 to resource
> flags for 64-bit memory addresses"), the OF PCI range parser takes a
> stricter view and treats 64-bit address ranges as advertised while
> before such ranges were treated as 32-bit.

Conceptually, I'm not sure why we need IORESOURCE_MEM_64 at all on
resources we get from DT.  I think the main point of IORESOURCE_MEM_64
is to convey the information that "this register, e.g., a PCI BAR, has
space for 64-bit values if you need to write to it."

When we're parsing this from DT, I think we're just getting a fixed
value and there's no concept of writing anything back, so it doesn't
seem like we should need to know how wide the hardware register is, or
even whether there *is* a hardware register.

But I'm sure the PCI resource allocation code probably depends on
IORESOURCE_MEM_64 in those host bridge windows in very ugly ways.

> A PCI-to-PCI bridges cannot forward 64-bit non-prefetchable memory
> ranges. As a result, the change in behaviour due to the commit causes
> allocation failure for devices that are connected behind PCI host
> bridges modelled as PCI-to-PCI bridge and require non-prefetchable bus
> addresses.
> 
> In order to not break platforms, override the 64-bit flag for
> non-prefetchable memory ranges that lie entirely below 4GB.
> 
> Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> Link: https://lore.kernel.org/r/7a1e2ebc-f7d8-8431-d844-41a9c36a8911@arm.com
> Signed-off-by: Punit Agrawal <punitagrawal@...il.com>
> Cc: Bjorn Helgaas <bhelgaas@...gle.com>
> Cc: Rob Herring <robh+dt@...nel.org>
> ---
>  drivers/pci/of.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> index da5b414d585a..b9d0bee5a088 100644
> --- a/drivers/pci/of.c
> +++ b/drivers/pci/of.c
> @@ -565,10 +565,14 @@ static int pci_parse_request_of_pci_ranges(struct device *dev,
>  		case IORESOURCE_MEM:
>  			res_valid |= !(res->flags & IORESOURCE_PREFETCH);
>  
> -			if (!(res->flags & IORESOURCE_PREFETCH))
> +			if (!(res->flags & IORESOURCE_PREFETCH)) {
>  				if (upper_32_bits(resource_size(res)))
>  					dev_warn(dev, "Memory resource size exceeds max for 32 bits\n");
> -
> +				if ((res->flags & IORESOURCE_MEM_64) && !upper_32_bits(res->end)) {
> +					dev_warn(dev, "Overriding 64-bit flag for non-prefetchable memory below 4GB\n");

Maybe "Clearing 64-bit flag"?

Can you include %pR, so we see the resource in question?

Unrelated but close by, would be nice if the preceding warning ("size
exceeds max") also included %pR.

> +					res->flags &= ~IORESOURCE_MEM_64;
> +				}
> +			}
>  			break;
>  		}
>  	}
> -- 
> 2.30.2
>