[PATCH] fpga zynq: Check the bitstream for validity

[Mike Looijmans]

On 09-11-16 16:56, Jason Gunthorpe wrote:
> On Wed, Nov 09, 2016 at 03:21:52PM +0100, Mike Looijmans wrote:
>> I think the basic idea behind the commit is flawed to begin with and the
>> patch should be discarded completely. The same discussion was done for the
>> Xilinx FPGA manager driver, which resulted in the decision that the tooling
>> should create a proper file.
>
> That hasn't changed at all, this just produces a clear and obvious
> message about what is wrong instead of 'timed out'.
>
> Remember, again, the Xilinx tools do not produce an acceptable
> bitstream file by default. You need to do very strange and special
> things to get something acceptable to this driver. It is a very easy
> mistake to make and hard to track down if you don't already know these
> details.
>
>> This driver should either become obsolete or at least move into the
>> same direction as the FPGA manager rather than away from that.
>
> I don't understand what you are talking about here, this is a fpga mgr
> driver already, and is consistent with the FPGA manger - the auto
> detect stuff was removed a while ago and isn't coming back.

That's exactly what I mean - the code was kept simple.

> It is perfectly reasonable for fpga manager drivers to pre-validate
> the proposed bitstream, IMHO all of the drivers should try and do
> that step.
>
> Remember, it is deeply unlikely but sending garbage to an FPGA could
> damage it.

Then what's the purpose of pre-validation? Sending valid data should be 
the normal case, not the exception.

>> Besides political arguments, there's a more pressing technical argument
>> against this theck as well: The whole check is pointless since the hardware
>> itself already verifies the validity of the stream.
>
> The purpose of the check is not to protect the hardware. The check is
> to help the user provide the correct input because the hardware
> provides no feedback at all on what is wrong with the input.
>
> And again, the out-of-tree Xilinx driver accepted files that this
> driver does not, so having a clear and understandable message is going
> to be very important for users.

Then just create a "validate my bitstream" tool.

I wrote a Python script to do what Xilinx refused to do years ago:
https://github.com/topic-embedded-products/meta-topic/blob/master/recipes-bsp/fpga/fpga-bit-to-bin/fpga-bit-to-bin.py
So apparently it wasn't hard to figure out what to do.

>> doesn't have any effect, the hardware will discard it. There's no reason to
>> waste CPU cycles duplicating this check in software.
>
> In the typical success case we are talking about 5 32 bit compares,
> which isn't even worth considering.

I'm mostly against the complication of the code. The code is more 
complex, and that's bad. It's firmware loading code, and it need not be 
aware of exactly what it is doing. I did not see any checks like this in 
other firmware loading code I've come across.

What you're creating here will require active maintenance.
There are already 7007 and 7012 devices which aren't in your list so the 
driver will refuse to load them until someone fills in the IDs.
The bitstream format is actually proprietary and undocumented. Any 
"checks" in code are likely based on guesswork and reverse engineering.
We also use partial reprogramming a lot. Did you test that? On all 
devices? And we're actually planning to go beyond that. Maybe I'll be 
providing parts of the data through ICAP and some through PCAP, that 
might even work, but the driver would block it for no apparent reason.


-- 
Mike Looijmans


Kind regards,

Mike Looijmans
System Expert

TOPIC Products
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