linux-kernel - Re: Question regarding "reserved-memory"

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Date:   Thu, 24 Oct 2019 09:51:04 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Ayan Halder <Ayan.Halder@....com>
Cc:     Mark Rutland <Mark.Rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Liviu Dudau <Liviu.Dudau@....com>,
        Mihail Atanassov <Mihail.Atanassov@....com>,
        "james qian wang (Arm Technology China)" <james.qian.wang@....com>,
        Brian Starkey <Brian.Starkey@....com>, nd <nd@....com>
Subject: Re: Question regarding "reserved-memory"

On Thu, Oct 24, 2019 at 9:22 AM Ayan Halder <Ayan.Halder@....com> wrote:
>
>
> Hi Folks,
>
> I have a question regarding "reserved-memory". I am using an Arm Juno
> platform which has a chunk of ram in its fpga. I intend to make this
> memory as reserved so that it can be shared between various devices
> for passing framebuffer.
>
> My dts looks like the following:-
>
> / {
>         .... // some nodes
>
>         tlx@...00000 {
>                 compatible = "simple-bus";
>                 ...
>
>                 juno_wrapper {
>
>                         ... /* here we have all the nodes */
>                             /* corresponding to the devices in the fpga */
>
>                         memory@...0000 {
>                                device_type = "memory";
>                                reg = <0x00 0x60000000 0x00 0x8000000>;
>                         };
>
>                         reserved-memory {
>                                #address-cells = <0x01>;
>                                #size-cells = <0x01>;
>                                ranges;
>
>                                framebuffer@...0000 {
>                                         compatible = "shared-dma-pool";
>                                         linux,cma-default;
>                                         reusable;
>                                         reg = <0x00 0x60000000 0x00 0x8000000>;
>                                         phandle = <0x44>;
>                                 };
>                         };
>                         ...
>                 }
>         }
> ...
> }
>
> Note that the depth of the "reserved-memory" node is 3.
>
> Refer __fdt_scan_reserved_mem() :-
>
>         if (!found && depth == 1 && strcmp(uname, "reserved-memory") == 0) {
>
>                 if (__reserved_mem_check_root(node) != 0) {
>                         pr_err("Reserved memory: unsupported node
> format, ignoring\n");
>                         /* break scan */
>                         return 1;
>                 }
>                 found = 1;
>
>                 /* scan next node */
>                 return 0;
>         }
>
> It expects the "reserved-memory" node to be at depth == 1 and so it
> does not probe it in our case.
>
> Niether from the
> Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
>  nor from commit - e8d9d1f5485b52ec3c4d7af839e6914438f6c285,
> I could understand the reason for such restriction.
>
> So, I seek the community's advice as to whether I should fix up
> __fdt_scan_reserved_mem() so as to do away with the restriction or
> put the "reserved-memory" node outside of 'tlx@...00000' (which looks
>  logically incorrect as the memory is on the fpga platform).

For now, I'm going to say it must be at the root level. I'd guess the
memory@...0000 node is also just ignored (wrong unit-address BTW).

I think you're also forgetting the later unflattened parsing of
/reserved-memory. The other complication IIRC is booting with UEFI
does it's own reserved memory table which often uses the DT table as
its source.

Rob