[RFC][PATCH 1/2] WIP: Devicetree bindings for Ion

[Rob Herring]

On Wed, Oct 7, 2015 at 5:36 AM, Andrew <andrew at ncrmnt.org> wrote:
> On 2015-10-07 02:01, Laura Abbott wrote:
>>
>> On 10/6/15 3:35 PM, Rob Herring wrote:
>>>
>>> On Tue, Oct 6, 2015 at 3:47 PM, Laura Abbott <labbott at fedoraproject.org>
>>> wrote:
>>>>
>>>> From: Laura Abbott <laura at labbott.name>
>>>>
>>>>
>>>> This adds a base set of devicetree bindings for the Ion memory
>>>> manager. This supports setting up the generic set of heaps and
>>>> their properties.
>>>>
>>>> Signed-off-by: Laura Abbott <laura at labbott.name>
>>>> Signed-off-by: Andrew Andrianov <andrew at ncrmnt.org>
>>>> ---
>>>>   drivers/staging/android/ion/devicetree.txt | 53
>>>> ++++++++++++++++++++++++++++++
>>>
>>>
>>> I have no issue with this going in here, but I do have lots of issues
>>> with this binding.
>>>
>>>>   1 file changed, 53 insertions(+)
>>>>   create mode 100644 drivers/staging/android/ion/devicetree.txt
>>>>
>>>> diff --git a/drivers/staging/android/ion/devicetree.txt
>>>> b/drivers/staging/android/ion/devicetree.txt
>>>> new file mode 100644
>>>> index 0000000..4a0c941
>>>> --- /dev/null
>>>> +++ b/drivers/staging/android/ion/devicetree.txt
>>>> @@ -0,0 +1,53 @@
>>>> +Ion Memory Manager
>>>> +
>>>> +Ion is a memory manager that allows for sharing of buffers via dma-buf.
>>>> +Ion allows for different types of allocation via an abstraction called
>>>> +a 'heap'. A heap represents a specific type of memory. Each heap has
>>>> +a different type. There can be multiple instances of the same heap
>>>> +type.
>>>> +
>>>> +Required properties for Ion
>>>> +
>>>> +- compatible: "linux,ion"
>>>> +
>>>> +All child nodes of a linux,ion node are interpreted as heaps
>>>> +
>>>> +required properties for heaps
>>>> +
>>>> +- linux,ion-heap-id: The Ion heap id used for allocation selection
>>>> +- linux,ion-heap-type: Ion heap type defined in ion.h
>>>> +- linux,ion-heap-name: Human readble name of the heap
>>>> +
>>>> +
>>>> +Optional properties
>>>> +- memory-region: A phandle to a memory region. Required for DMA heap
>>>> type
>>>> +(see reserved-memory.txt for details on the reservation)
>>>> +- linux,ion-heap-align: Alignment for the heap.
>>>> +
>>>> +Example:
>>>> +
>>>> +       ion {
>>>> +               compatbile = "linux,ion";
>>>> +               #address-cells = <1>;
>>>> +               #size-cells = <0>;
>>>> +
>>>> +               ion-system-heap {
>>>> +                       linux,ion-heap-id = <0>;
>>>> +                       linux,ion-heap-type = <ION_SYSTEM_HEAP_TYPE>;
>>>> +                       linux,ion-heap-name = "system";
>>>
>>>
>>> How does this vary across platforms? Is all of this being pushed down
>>> to DT, because there is no coordination of this at the kernel ABI
>>> level across platforms. In other words, why can't heap 0 be hardcoded
>>> as system heap in the driver. It seems to me any 1 of these 3
>>> properties could be used to derive the other 2.
>>>
>>
>> Right now there is no guarantee heap IDs will be the same across
>> platforms. The heap IDs are currently part of the userspace ABI
>> as well since userspace clients must pass in a mask of the heap
>> IDs to allocate from. If we assume all existing clients could change,
>> heaps such as the system heap could be mandated to have the same
>> heap ID but we'd still run into problems if you have multiple
>> heaps of the same type (e.g. multiple carveouts)

Vendors largely ignore the kernel-userspace ABI and anything in
staging is not a ABI. So arguments about what the ABI is currently is
pointless IMO.

Pushing an inconsistent kernel ABI to DT is not the answer.

>
> I don't really like the idea of enforcing any IDs here. As of now
> heap ids are generally something VERY platform-specific
> (or even product-specific). Personally I'd prefer something like this
> for userspace apps:
>
> int id1 = ion_get_heap_id("camera");
> if (id1 < 0) {
>       fprintf(stderr, "Invalid heap id");
>       exit(1);
> }
>
> int id2 = ion_get_heap_id("backup-heap");
> if (id2 < 0) {
>       fprintf(stderr, "Invalid heap id");
>       exit(1);
> }

We've learned that creating number spaces like this are bad (irqs,
gpios, /dev nodes). We should move away from that. Why should
userspace care about IDs or what the IDs are? An ID is just encoding
certain implicit requirements. So are the strings here. Users should
express what capabilities, restrictions, etc. they have, and then the
kernel can find the best heap.

> ...
>
> int ret = ion_alloc(fd, 0x100, 0x4,
>               (id1 | id2),
>               0, &handle);
>
>
> What concerns kernel stuff, things are simpler - we may just pass the heap
> to use
> by a reference in devicetree node for that driver. Something like that:
>
> ...
>                ion-decoder-region : region_ddr {
>                        linux,ion-heap-id = <1>;
>                        linux,ion-heap-type = <ION_DMA_HEAP_TYPE>;
>                        linux,ion-heap-name = "decoder_mem"
>                        memory-region = <&camera_region>;
>                 };
> ...
>                 video_decoder at 80180000 {
>                         compatible = "acme,h266dec";
>                         reg = <0x80180000 0x20000>,
>                         reg-names = "registers";
>                         interrupts = <12>;
>                         interrupt-parent = <&vic1>;
>                         ion-heaps-for-buffers = <&ion-decoder-region>

This is how memory-region is supposed to work. I don't see why we need
an additional level of indirection.

Rob