Re: [RFC PATCH] fpga: Introduce new fpga subsystem

[Alan Tull <atull@altera.com>]

On Wed, 2013-09-18 at 14:32 -0600, Jason Gunthorpe wrote:
> On Wed, Sep 18, 2013 at 03:15:17PM -0400, Jason Cooper wrote:
> 
> > + Jason Gunthorpe
> 
> Thanks, looks interesting, we could possibly use this interface if it 
> met our needs..
>  
> > On Wed, Sep 18, 2013 at 05:56:39PM +0200, Michal Simek wrote:
> > > This new subsystem should unify all fpga drivers which
> > > do the same things. Load configuration data to fpga
> > > or another programmable logic through common interface.
> > > It doesn't matter if it is MMIO device, gpio bitbanging,
> > > etc. connection. The point is to have the same
> > > inteface for these drivers.
> 
> So, we have many years of in-field experience with this and this API
> doesn't really match what we do.
> 
> Here are the steps we perform, from userspace:
>  - Ask kernel to place FPGA into reset and prepare for programming
>    * Kernel can return an error (eg FPGA failed to erase, etc)
>    * this is the PROG_N low -> DONE high, PROG_N high -> INIT_N high
>      sequencing on Xilinx chips
>  - Ask kernel to load a bitstream.
>    * Userspace locats the bitstream file to load, and the mmaps it.
>    * Userspace passes the entire file in a single write() call to the
>      kernel which streams it over the configuration bus
>    * The kernel can report an erro rhere (eg Xilinx can report CRC
>    error)
>  - Ask the kernel to verify that configuration is complete. 
>    * On Xilinx this wait for done to go high
>  - Ask the kernel to release the configuration bus (tristate
>    all drivers) (or sometimes we have to drive the bus low,
>    it depends on the bitfile, user space knows what to do)
> 
> It is very important that userspace know exactly which step fails
> because the resolution is different. We use this in a manufacturing
> setting, so failures are expected and need quick root cause
> determination.
> 
> You could probably address that need by very clearly defining a
> variety of errno values for the various cases. However, it would be a
> disaster if every driver did something a little different :|
> 
> Using request_firmware exclusively is not useful for us. We
> format the bitfile with a header that contains our internal tracking
> information. Sometimes we need to bitswap the bitfile. Our userspace
> handles all of this and can pass a bitfile in memory to write().
> 
> request_firmware would be horrible to use :)
> 
> Our API uses a binary sysfs attribute to stream the FPGA data, you
> might want to consider that.
> 
> Regards,
> Jason

The firmware approach is interesting.  It might be less flexible
compared with my original code (see link to git below) that this is
based on.  The original code created a devnode like /dev/fpga0 and a raw
bitstream could be loaded by doing 'cat bitstream > /dev/fpga0'.  Or
some other userspace app could write the /dev/fpga0 to handle any
headers that needed to be added to the bitstream.

This code also creates a set of files under /sys for each separate fpga.
I.e. checking status by looking at /sys/class/fpga/fpag0/status.  It
would be pretty small changes to control reseting the fpga by adding a
'reset' file there also (added first to the framework, and an interface
into the low level fpga manager driver).

I am trying this out with my low level fpga manager driver.  I'm very
curious about your approach and I am wondering whether the firmware
approach will work for us or not.

Will this framework handle more than one fpga at a time?

Is there some way a per-device userspace helper can be added that can
handle adding the headers?  Such that different fpga types get different
helpers?

My fpga framework code is in git at:
http://rocketboards.org/gitweb/?p=linux-socfpga.git;a=commit;h=7b7c04ef3f8589349211bdfe884e42d6b7554b27

and

http://rocketboards.org/gitweb/?p=linux-socfpga.git;a=commit;h=57ee6197d65015620cd6aad435a695ce00a48a8c

Best Regards,
Alan

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