linux-kernel - Re: [RFC 7/8] fpga-region: add sysfs interface

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Date:   Wed, 22 Feb 2017 10:52:12 -0600
From:   Alan Tull <delicious.quinoa@...il.com>
To:     Moritz Fischer <moritz.fischer@...us.com>
Cc:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
        "Nadathur, Sundar" <sundar.nadathur@...el.com>,
        Yves Vandervennet <yves.vandervennet@...ux.intel.com>,
        "matthew.gerlach@...ux.intel.com" <matthew.gerlach@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "linux-fpga@...r.kernel.org" <linux-fpga@...r.kernel.org>,
        "Marek Va??ut" <marex@...x.de>
Subject: Re: [RFC 7/8] fpga-region: add sysfs interface

On Wed, Feb 22, 2017 at 10:44 AM, Moritz Fischer
<moritz.fischer@...us.com> wrote:
> On Wed, Feb 22, 2017 at 8:33 AM, Alan Tull <delicious.quinoa@...il.com> wrote:
>> On Tue, Feb 21, 2017 at 11:38 PM, Moritz Fischer
>> <moritz.fischer@...us.com> wrote:
>>
>> Hi Moritz,
>>
>>> Hi all,
>>>
>>> On Tue, Feb 21, 2017 at 9:12 PM, Jason Gunthorpe
>>> <jgunthorpe@...idianresearch.com> wrote:
>>>> On Tue, Feb 21, 2017 at 07:49:19PM -0800, Moritz Fischer wrote:
>>>>
>>>>> fdt does this out of the box, too. So far I've seen nothing fdt
>>>>> couldn't do (or doesn't do let's rather say).
>>>>
>>>> tlv/fdt/http headers are all essentially exactly the same
>>>> thing. Key/value pairs with various encoding schemes.
>>>>
>>>> I don't think we don't need a tree of data, so fdt is overkill.
>>>>
>>>> tlv is not substantially easier to parse correctly than the
>>>> structured plain text headers.. It is just in binary so it can
>>>> represent binary-ish things better.
>>>
>>> TLV Seems easy enough. To give an update, I played with fdt a bit to see
>>> how far I get in half an hour. I got bool / int / strings to work
>>> quite fast (~30mins).
>>
>> Thanks for doing this fast piece of exploratory coding.  It does
>> confirm that for Linux, using fdt is pretty straightforward here.
>> However...
>>
>>> Please disregard the horrible hackyness of this ...
>>>
>>> For simplicity I stuck the header on top of my bitfile with:
>>>
>>> <snip>
>>> /dts-v1/;
>>>
>>> /{
>>>         description = "Test";
>>>         compressed = <0>;
>>>         encrypted = <1>;
>>> };
>>
>> I understand that this is a simplified example, but it looks a lot
>> like KVP which then gets compiled by dtc.
>>
>> If we do KVP or TLV we get skip using dtc, which would be nice for non-dt
>> OS's using the same images.
>
> I used dtc for pure lazyness. Writing a blob to a file using libfdt is
> about as much
> code as parsing it.

Thanks for that clarification.  I haven't used libfdt myself.  That
takes care of the license issue I brought up below.

I have heard that MS is averse to using DT, but I'm not clear
about why other than that it isn't native to Windows already.

Alan

> Even with KVP or TLV you have some code that needs
> to encode / pack your header into a file.
>
> libfdt has an example that creates an empty tree. Write that to a file, done.
>
> 1: Create empty tree
> https://github.com/dgibson/dtc/blob/master/libfdt/fdt_empty_tree.c
>
> 2: fopen / fwrite, done
>
>> Also, the license of libfdt allows the use by proprietary
>> os's, but that's not true for dtc.
>
> Why would that be an issue, you don't need to link anything to run
> dtc. That being
> said as I pointed out above you don not have to actually use dtc if the values
> are known ahead of time (like in our case). What you'd get from using dtc is to
> encode arbitrary values (for the types supported).
>
> Cheers,
>
> Moritz