RE: [PATCH v2 19/22] fpga: intel: afu: add header sub feature support

["Wu, Hao" <hao.wu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

>  On Sun, Jun 25, 2017 at 8:52 PM, Wu Hao <hao.wu@intel.com> wrote:
> 
> Hi Hao,
> 
> > The header register set is always present for the Port/AFU, it is mainly
> > for capability, control and status of the ports that AFU connected to.
> 
> So just to be clear, the reset function is acting on the Port, not the
> AFU, right?

Hi Alan

AFU will be reset as well. Once port reset is issued, a compliant HW
AFU will reset all its state and stop sending requests, and HW will set
port reset ack bit when all outstanding requests initiated have been
drained.

Will add some notes here.

> 
> >
> > This patch implements header sub feature support.
> 
> Please add a brief reminder here what the 'header' is.  It's defined
> in patch 7 as being part of the feature list, but hardly mentioned
> when I grep intel-fpga.txt.

Sure, will adds some notes here.

Actually the header sub feature means the registers belong to the
feature device (e.g port and FME), not any sub features (e.g PR, 
Power management).

> 
> > Below user interfaces
> > are created by this patch.
> >
> > Sysfs interface:
> > * /sys/class/fpga/<fpga.x>/<intel-fpga-port.x>/id
> >   Read-only. Port ID.
> >
> > Ioctl interface:
> > * FPGA_PORT_RESET
> >   Reset the FPGA AFU Port.
> >
> > Signed-off-by: Tim Whisonant <tim.whisonant@intel.com>
> > Signed-off-by: Enno Luebbers <enno.luebbers@intel.com>
> > Signed-off-by: Shiva Rao <shiva.rao@intel.com>
> > Signed-off-by: Christopher Rauer <christopher.rauer@intel.com>
> > Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
> > Signed-off-by: Wu Hao <hao.wu@intel.com>
> > ---
> > v2: add sysfs documentation.
> > ---
> >  .../ABI/testing/sysfs-platform-intel-fpga-afu      |  7 ++++
> >  drivers/fpga/intel-afu-main.c                      | 44 +++++++++++++++++++++-
> >  include/uapi/linux/intel-fpga.h                    | 14 +++++++
> >  3 files changed, 64 insertions(+), 1 deletion(-)
> >  create mode 100644 Documentation/ABI/testing/sysfs-platform-intel-fpga-
> afu
> >
> > diff --git a/Documentation/ABI/testing/sysfs-platform-intel-fpga-afu
> b/Documentation/ABI/testing/sysfs-platform-intel-fpga-afu
> > new file mode 100644
> > index 0000000..8ad22c9
> > --- /dev/null
> > +++ b/Documentation/ABI/testing/sysfs-platform-intel-fpga-afu
> > @@ -0,0 +1,7 @@
> > +What:          /sys/bus/platform/devices/intel-fpga-port.0/id
> > +Date:          June 2017
> > +KernelVersion:  4.12
> > +Contact:       Wu Hao <hao.wu@intel.com>
> > +Description:   Read-only. It returns id of this port. One Intel FPGA device
> > +               may have more than one port. Userspace could use this id to
> > +               distinguish different ports under same FPGA device.
> > diff --git a/drivers/fpga/intel-afu-main.c b/drivers/fpga/intel-afu-main.c
> > index 96d0367..2a17cde 100644
> > --- a/drivers/fpga/intel-afu-main.c
> > +++ b/drivers/fpga/intel-afu-main.c
> > @@ -18,25 +18,66 @@
> >
> >  #include <linux/kernel.h>
> >  #include <linux/module.h>
> > +#include <linux/intel-fpga.h>
> >
> >  #include "intel-feature-dev.h"
> >
> > +static ssize_t
> > +id_show(struct device *dev, struct device_attribute *attr, char *buf)
> > +{
> > +       int id = fpga_port_id(to_platform_device(dev));
> > +
> > +       return scnprintf(buf, PAGE_SIZE, "%d\n", id);
> > +}
> > +static DEVICE_ATTR_RO(id);
> > +
> > +static const struct attribute *port_hdr_attrs[] = {
> > +       &dev_attr_id.attr,
> > +       NULL,
> > +};
> > +
> >  static int port_hdr_init(struct platform_device *pdev, struct feature *feature)
> >  {
> >         dev_dbg(&pdev->dev, "PORT HDR Init.\n");
> >
> > -       return 0;
> > +       fpga_port_reset(pdev);
> 
> So the port will be reset here, which happens during fme_probe().
> IIUC the PR region is empty then, there is just the static region,
> right?

port_hdr_init is invoked during afu_probe() function. The fpga_port_reset
only resets the AFU's state and not empty the PR region. User doesn't need
to program it again after port reset.

The purpose of this reset in port_hdr_init function, is to make sure that we 
could have a clean start whenever the port driver module is loaded. And
similar as the one added in afu release.

> 
> > +
> > +       return sysfs_create_files(&pdev->dev.kobj, port_hdr_attrs);
> 
> Greg wrote an article that there could be a race condition caused by
> creating sysfs files this late [1] and I see sysfs_create_files() used
> very sparingly in the kernel.  I'm thinking that fpga-bridge should
> provide a place to create sysfs files earlier by adding an
> attribute_group to fpga_bridge_ops (same for fpga-mgr and fpga-region)
> and then fpga_bridge_register could do bridge->dev.groups =
> br_ops->groups.  I'll put a patch for that out soon.
> 

Hm... I understand there could be a race condition if creates sysfs files late.
Actually the reasons I prefer to have each sub feature to create its own sysfs
files are, 1) if any sub feature is not present, then related init function won't 
be invoked and related sysfs files won't be created at all. Then end user could
know which sub features are present by checking these sysfs nodes easily. 
2) Another point of view is about extension of each sub feature, for example,
Some new registers introduced when sub feature's revision > n, so each sub
feature's init function could check the sub feature's revision to decide which
sysfs interfaces are needed. I think this should be a flexible way for extension.

> >  }
> >
> >  static void port_hdr_uinit(struct platform_device *pdev,
> >                                         struct feature *feature)
> >  {
> >         dev_dbg(&pdev->dev, "PORT HDR UInit.\n");
> > +
> > +       sysfs_remove_files(&pdev->dev.kobj, port_hdr_attrs);
> > +}
> > +
> > +static long
> > +port_hdr_ioctl(struct platform_device *pdev, struct feature *feature,
> > +                                       unsigned int cmd, unsigned long arg)
> > +{
> > +       long ret;
> > +
> > +       switch (cmd) {
> > +       case FPGA_PORT_RESET:
> > +               if (!arg)
> > +                       ret = fpga_port_reset(pdev);
> 
> fpga_port_reset() disables and reenables traffic on the port.  Is
> there ever a time when that would be unsafe to do?  Like while DMA is
> happening?  When I see a function called 'reset' exposed to userspace,
> I become concerned that hitting that reset at the wrong time could
> cause problems.  We've discussed this some, but could you please
> remind me when userspace would need to reset the port?  Please add
> documentation of what the intended use of this ioctl would be, when it
> is valid to request a reset from userspace and when userspace should
> never do that.

Sure, I will add some more description in uapi header file on this IOCTL.

Per my understanding, this API could be invoked at any time, even there
is some DMA operation or PR operation at the same time. It should not
cause any system level problem, but may cause functional failures ( e.g
DMA or PR operation failure) and it should be recoverable.

> 
> The pcie code, the AFU file interface, and the bridge code all do port
> reset.  This code is spread out over a few patches, but I'll comment
> here for now.  I'm trying to keep track of everything that resets the
> port.  The port gets reset in afu_probe, fme_probe, the AFU file
> release, and intel-pcie.c after parsing the features.  Also the port
> is esentially reset after doing reprogramming, since that involves a
> bridge disable/enable.   In the v1 review, the issue raised that the
> port functionality could be an expansion of fpga-bridge.  If reset
> were added to the fpga_bridge_ops and a fpga_bridge_reset API added to
> fpga-bridge, then anything in the kernel that owns the bridge could
> reset it.  That is of course assuming that this code doesn't need to
> reset the port before the fpga-bridge is created.

Actually in the pcie code, it needs to enable fpga port (put it out of reset,
as port is in reset by default), otherwise the AFU MMIO region is not
accessible. It's only a one time action.

For the AFU interface, it gives application a way to reset the AFU if
anything goes wrong or application wants to make AFU back to 
default state.

For the bridge code, it just makes sure port is in reset during PR, which
is requested by the PR flow of Intel FPGA device.

As you know, the fpga-bridge is created under FME module now, and
port/AFU is another module which could be turned into a VF and assigned
to a virtual machine, even we added a reset ops to fpga-bridge, we still
need an interface to do port reset as fpga-bridges (and FME) are always
in PF in host.  :)

Thanks
Hao

> 
> Thanks,
> Alan Tull
> 
> [1] http://www.kroah.com/log/blog/2013/06/26/how-to-create-a-sysfs-file-
> correctly/
> 
> > +               else
> > +                       ret = -EINVAL;
> > +               break;
> > +       default:
> > +               dev_dbg(&pdev->dev, "%x cmd not handled", cmd);
> > +               ret = -ENODEV;
> > +       }
> > +
> > +       return ret;
> >  }
> >
> >  struct feature_ops port_hdr_ops = {
> >         .init = port_hdr_init,
> >         .uinit = port_hdr_uinit,
> > +       .ioctl = port_hdr_ioctl,
> >  };
> >
> >  static struct feature_driver port_feature_drvs[] = {
> > @@ -76,6 +117,7 @@ static int afu_release(struct inode *inode, struct file
> *filp)
> >
> >         dev_dbg(&pdev->dev, "Device File Release\n");
> >
> > +       fpga_port_reset(pdev);
> >         feature_dev_use_end(pdata);
> >         return 0;
> >  }
> > diff --git a/include/uapi/linux/intel-fpga.h b/include/uapi/linux/intel-fpga.h
> > index be295ae..be5f813 100644
> > --- a/include/uapi/linux/intel-fpga.h
> > +++ b/include/uapi/linux/intel-fpga.h
> > @@ -30,8 +30,11 @@
> >  #define FPGA_MAGIC 0xB6
> >
> >  #define FPGA_BASE 0
> > +#define PORT_BASE 0x40
> >  #define FME_BASE 0x80
> >
> > +/* Common IOCTLs for both FME and AFU file descriptor */
> > +
> >  /**
> >   * FPGA_GET_API_VERSION - _IO(FPGA_MAGIC, FPGA_BASE + 0)
> >   *
> > @@ -50,6 +53,17 @@
> >
> >  #define FPGA_CHECK_EXTENSION   _IO(FPGA_MAGIC, FPGA_BASE + 1)
> >
> > +/* IOCTLs for AFU file descriptor */
> > +
> > +/**
> > + * FPGA_PORT_RESET - _IO(FPGA_MAGIC, PORT_BASE + 0)
> > + *
> > + * Reset the FPGA AFU Port. No parameters are supported.
> > + * Return: 0 on success, -errno of failure
> > + */
> > +
> > +#define FPGA_PORT_RESET                _IO(FPGA_MAGIC, PORT_BASE + 0)
> > +
> >  /* IOCTLs for FME file descriptor */
> >
> >  /**
> > --
> > 1.8.3.1
> >