linux-kernel - Question regarding "reserved-memory"

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Date:   Thu, 24 Oct 2019 14:22:17 +0000
From:   Ayan Halder <Ayan.Halder@....com>
To:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        Mark Rutland <Mark.Rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "m.szyprowski@...sung.com" <m.szyprowski@...sung.com>
CC:     "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: Question regarding "reserved-memory"


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).


Thanks,
Ayan