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

Pavel Machek
2013-09-19T10:03:42Z

Hi!

> > + 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 :|

Well, exact steps are a bit hw-specific, too. So, if anything, I'd
suggest standartizing the errno values.

> 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().

Take a look at
https://www.kernel.org/doc/Documentation/firmware_class/README . You
can do processing at

4), userspace:
- hotplug: cat appropriate_firmware_image > \
/sys/class/firmware/xxx/data

What is the specific reason request_firmware is unsuitable?

Thanks,
Pavel
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