Re: [PATCH v1 3/7] fpga: dfl: Allow for ports with no local bar space.

From: Tom Rix
Date: Mon Feb 21 2022 - 13:01:04 EST


On 2/21/22 9:22 AM, matthew.gerlach@xxxxxxxxxxxxxxx wrote:


On Fri, 18 Feb 2022, Tom Rix wrote:


On 2/17/22 11:31 PM, Zhang, Tianfei wrote:

-----Original Message-----
From: Tom Rix <trix@xxxxxxxxxx>
Sent: Tuesday, February 15, 2022 11:06 PM
To: Zhang, Tianfei <tianfei.zhang@xxxxxxxxx>; Wu, Hao <hao.wu@xxxxxxxxx>;
mdf@xxxxxxxxxx; Xu, Yilun <yilun.xu@xxxxxxxxx>; linux-fpga@xxxxxxxxxxxxxxx;
linux-doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Cc: corbet@xxxxxxx; Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
Subject: Re: [PATCH v1 3/7] fpga: dfl: Allow for ports with no local bar space.


On 2/14/22 3:26 AM, Tianfei zhang wrote:
From: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>

  From a fpga partial reconfiguration standpoint, a port may not be
connected any local BAR space.  The port could be connected to a
different PCIe Physical Function (PF) or Virtual Function (VF), in
which case another driver instance would manage the endpoint.
It is not clear if this is part of iofs or a bug fix.
This is the new implementation/feature of IOFS.
On IOFS support multiple methods to access the AFU.
1. Legacy Model. This is used for N3000 and N5000 card.
In this model the entire AFU region is a unit of PR, and there is a Port device connected to this AFU.
On DFL perspective, there is "Next AFU" point to the AFU, and the "BarID" is  the PCIe Bar ID of AFU.
In this model, we can use the AFU APIs to access the entire AFU resource, like MMIO.
2. Micro-Personas in AFU.
IOFS intruding new model for PR and AFU access.
Micro-Personas allow the RTL developer to designate their own AFU-defined PR regions.
In this model the unit of PR is not the entire AFU, instead
the unit of PR can be any size block or blocks inside the AFU.
3. Multiple VFs per PR slot.
In this method, we can instance multiple VFs over SRIOV for one PR slot, and access the AFU resource
by different VFs in virtualization usage. In this case, the Port device would not connected to AFU (the BarID of Port device
should be set to invalid), so this patch want to support this use model.

What I am looking for is how the older cards using (my term) dfl 1 will still work with dfl 2 and vice versa.

No where do I see a version check for dfl 2 nor a pci id check so either this just works or backward compatibility has not be considered.

Please add a backward compatibility section to the doc patch


Signed-off-by: Matthew Gerlach <matthew.gerlach@xxxxxxxxxxxxxxx>
Signed-off-by: Tianfei Zhang <tianfei.zhang@xxxxxxxxx>
---
   drivers/fpga/dfl-pci.c | 8 ++++++++
   1 file changed, 8 insertions(+)

diff --git a/drivers/fpga/dfl-pci.c b/drivers/fpga/dfl-pci.c index
4d68719e608f..8abd9b408403 100644
--- a/drivers/fpga/dfl-pci.c
+++ b/drivers/fpga/dfl-pci.c
@@ -243,6 +243,7 @@ static int find_dfls_by_default(struct pci_dev *pcidev,
           v = readq(base + FME_HDR_CAP);
           port_num = FIELD_GET(FME_CAP_NUM_PORTS, v);

+        dev_info(&pcidev->dev, "port_num = %d\n", port_num);
           WARN_ON(port_num > MAX_DFL_FPGA_PORT_NUM);

           for (i = 0; i < port_num; i++) {
@@ -258,6 +259,13 @@ static int find_dfls_by_default(struct pci_dev *pcidev,
                */
               bar = FIELD_GET(FME_PORT_OFST_BAR_ID, v);
               offset = FIELD_GET(FME_PORT_OFST_DFH_OFST, v);
+            if (bar >= PCI_STD_NUM_BARS) {
Is bar set to a better magic number that pci_std_num_bars ? maybe 0xff's

How do you tell between this case and broken hw ?
Yes, I agree that magic number is better, Currently the RTL using PCI_STD_NUM_BARS for an invalid PCIe bar number.

How do you tell between this case and broken hw ?

Tom

The field, FME_PORT_OFST_BAR_ID, is a three bit field, which is pretty common for BARs on PCI.  PCI_STD_NUM_BARS is defined as 6. Current HW implementations are filing this field with the value, 7, which is close to the suggestion of 0xff's.  How about we define the following magic number?
#define NO_LOCAL_PORT_BAR    7

We should also change the dev_info to be a dev_dbg and more precise to something like the following:

    if (bar == NO_LOCAL_PORT_BAR) {
        dev_dbg(&pcidev->dev, "No local port BAR space.\n");
        continue;
    }

What I am looking for is way generally is to tell if this is an old framework or a new one.

Maybe a flag and/or version added to dfl_fpga_cdev on probing ?

(The meaning of released_port_num likely needs to change there as well)

So in this case you could check if this was the new framework before doing the bar check.

Similar checks other places where ofs stuff is being fit it.

My concern is the fitting in without checking will break the old stuff.

And why I wanted to see a probing writeup in the dfl.rst doc

Tom



Move up a line and skip getting an offset that will not be used.
Yes, this line is not necessary, I will remove it on next version patch.

+ dev_info(&pcidev->dev, "skipping port without
local BAR space %d\n",
+                     bar);
+                continue;
+            } else {
+                dev_info(&pcidev->dev, "BAR %d offset %u\n",
bar, offset);
+            }
               start = pci_resource_start(pcidev, bar) + offset;
               len = pci_resource_len(pcidev, bar) - offset;

Is similar logic needed for else-if (port) block below this ?
I think, the else-if is not necessary. I will remove it on next version patch.
Tom