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Date:   Mon, 30 Nov 2020 16:45:20 -0800 (PST)
From:   matthew.gerlach@linux.intel.com
To:     "Wu, Hao" <hao.wu@intel.com>
cc:     "linux-fpga@vger.kernel.org" <linux-fpga@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "mdf@kernel.org" <mdf@kernel.org>,
        "trix@redhat.com" <trix@redhat.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "corbet@lwn.net" <corbet@lwn.net>
Subject: RE: [PATCH v3 2/2] fpga: dfl: look for vendor specific capability
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On Sat, 28 Nov 2020, Wu, Hao wrote:

>> Subject: [PATCH v3 2/2] fpga: dfl: look for vendor specific capability
>
> Maybe we can change the title a little bit, what about
> fpga: dfl-pci: locate DFLs by PCIe vendor specific capability
>
>>
>> From: Matthew Gerlach <matthew.gerlach@linux.intel.com>
>>
>> A DFL may not begin at offset 0 of BAR 0.  A PCIe vendor
>> specific capability can be used to specify the start of a
>> number of DFLs.
>
> A PCIe vendor specific extended capability is introduced by Intel to
> specify the start of a number of DFLs.

Your suggestion is more precise.
>
>
>>
>> Signed-off-by: Matthew Gerlach <matthew.gerlach@linux.intel.com>
>> ---
>> v3: Add text and ascii art to documentation.
>>     Ensure not to exceed PCIe config space in loop.
>>
>> v2: Update documentation for clarity.
>>     Clean up  macro names.
>>     Use GENMASK.
>>     Removed spurious blank lines.
>>     Changed some calls from dev_info to dev_dbg.
>>     Specifically check for VSEC not found, -ENODEV.
>>     Ensure correct pci vendor id.
>>     Remove check for page alignment.
>>     Rename find_dfl_in_cfg to find_dfls_by_vsec.
>>     Initialize target memory of pci_read_config_dword to invalid values before
>> use.
>> ---
>>  Documentation/fpga/dfl.rst | 25 +++++++++++
>>  drivers/fpga/dfl-pci.c     | 91 +++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 115 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/fpga/dfl.rst b/Documentation/fpga/dfl.rst
>> index 0404fe6ffc74..fa0da884a818 100644
>> --- a/Documentation/fpga/dfl.rst
>> +++ b/Documentation/fpga/dfl.rst
>> @@ -501,6 +501,31 @@ Developer only needs to provide a sub feature
>> driver with matched feature id.
>>  FME Partial Reconfiguration Sub Feature driver (see drivers/fpga/dfl-fme-
>> pr.c)
>>  could be a reference.
>>
>> +Location of DFLs on a PCI Device
>> +===========================
>> +There are two ways of locating DFLs on a PCI Device.  The original
>
> I found this new VSEC is only for PCIe device, correct? If so, let's make
> sure descriptions are accurate. E.g. default method for all devices
> and a new method for PCIe device.

Yes, the default method can be used with PCI and PCIe device, and the VSEC 
approach is PCIe, only.  Documentation can be made more precise.

>
>> +method assumed the start of the first DFL to offset 0 of bar 0.
>> +If the first node of the DFL is an FME, then further DFLs
>> +in the port(s) are specified in FME header registers.
>> +Alternatively, a vendor specific capability structure can be used to
>> +specify the location of all the DFLs on the device, providing flexibility
>> +for the type of starting node in the DFL.  Intel has reserved the
>> +VSEC ID of 0x43 for this purpose.  The vendor specific
>> +data begins with a 4 byte vendor specific register for the number of DFLs
>> followed 4 byte
>> +Offset/BIR vendor specific registers for each DFL. Bits 2:0 of Offset/BIR
>> register
>
> Do we have a defined register name here? or it's named as Offset/BIR register?
> Sounds a little wired, and I see you defined it as DFLS_RES?

The Offset/BIR terminology is also used in the MSI-X capability structure.

>
>> +indicates the BAR, and bits 31:3 form the 8 byte aligned offset where bits
>> 2:0 are
>> +zero.
>> +
>> +        +----------------------------+
>> +        |31     Number of DFLS      0|
>> +        +----------------------------+
>> +        |31     Offset     3|2 BIR  0|
>> +        +----------------------------+
>> +                      . . .
>> +        +----------------------------+
>> +        |31     Offset     3|2 BIR  0|
>> +        +----------------------------+
>> +
>
> Maybe it's better to have register name and offset in above table.
> BTW: if there is some public link to related spec, that helps too.

I'll consider adding a register name and offset, but I am not sure it adds 
much information.

>
>>
>>  Open discussion
>>  ===============
>> diff --git a/drivers/fpga/dfl-pci.c b/drivers/fpga/dfl-pci.c
>> index b27fae045536..a58bf4299d6b 100644
>> --- a/drivers/fpga/dfl-pci.c
>> +++ b/drivers/fpga/dfl-pci.c
>> @@ -27,6 +27,14 @@
>>  #define DRV_VERSION	"0.8"
>>  #define DRV_NAME	"dfl-pci"
>>
>> +#define PCI_VSEC_ID_INTEL_DFLS 0x43
>> +
>> +#define PCI_VNDR_DFLS_CNT 8
>> +#define PCI_VNDR_DFLS_RES 0x0c
>
> They are both register offset? it's better to use the same style.
>
> 0x8
> 0xc
>
> Something like this.

Agrreed.
>
>> +
>> +#define PCI_VNDR_DFLS_RES_BAR_MASK GENMASK(2, 0)
>> +#define PCI_VNDR_DFLS_RES_OFF_MASK GENMASK(31, 3)
>> +
>>  struct cci_drvdata {
>>  	struct dfl_fpga_cdev *cdev;	/* container device */
>>  };
>> @@ -119,6 +127,84 @@ static int *cci_pci_create_irq_table(struct pci_dev
>> *pcidev, unsigned int nvec)
>>  	return table;
>>  }
>>
>> +static int find_dfls_by_vsec(struct pci_dev *pcidev, struct
>> dfl_fpga_enum_info *info)
>> +{
>> +	u32 bar, offset, vndr_hdr, dfl_cnt, dfl_res;
>> +	int dfl_res_off, i, voff = 0;
>> +	resource_size_t start, len;
>> +
>> +	if (pcidev->vendor != PCI_VENDOR_ID_INTEL)
>> +		return -ENODEV;
>
> Check it later then other vendor can add their ones easily?
>
>> +
>> +	while ((voff = pci_find_next_ext_capability(pcidev, voff,
>> PCI_EXT_CAP_ID_VNDR))) {
>> +		vndr_hdr = 0;
>
> It seems it doesn't need this.

Initializing vndr_hdr = 0 ensures a failed pci_read_config_dword() failure 
is handled properly.  I will remove the call and the debug information 
anyway.

  >
>> +		pci_read_config_dword(pcidev, voff + PCI_VNDR_HEADER,
>> &vndr_hdr);
>> +
>> +		dev_dbg(&pcidev->dev,
>> +			"vendor-specific capability id 0x%x, rev 0x%x len
>> 0x%x\n",
>> +			PCI_VNDR_HEADER_ID(vndr_hdr),
>> +			PCI_VNDR_HEADER_REV(vndr_hdr),
>> +			PCI_VNDR_HEADER_LEN(vndr_hdr));
>
> Suggest remove this debug information.
>
>> +
>> +		if (PCI_VNDR_HEADER_ID(vndr_hdr) ==
>> PCI_VSEC_ID_INTEL_DFLS)
>
> How about
> 		if (vendor == intel && header_id == INTEL_DFLS)
> 			break;
Seems reasonable.
>
>> +			break;
>> +	}
>> +
>> +	if (!voff) {
>> +		dev_dbg(&pcidev->dev, "%s no VSEC found\n", __func__);
>
> No DFL VSEC found
>
> There could be many different VSECs but no DFL ones.

Agreed.
>
>> +		return -ENODEV;
>> +	}
>> +
>> +	dfl_cnt = 0;
>
> Can be merged into the line which defines dfl_cnt? Or we can just
> remove this line.

This initialization ensures that a failure to the pci_read_config_dword 
function below is handled properly.  It can be merged into the definition 
line.

>
>> +	pci_read_config_dword(pcidev, voff + PCI_VNDR_DFLS_CNT,
>> &dfl_cnt);
>> +	dev_dbg(&pcidev->dev, "dfl_cnt %d\n", dfl_cnt);
>> +	for (i = 0; i < dfl_cnt; i++) {
>> +		dfl_res_off = voff + PCI_VNDR_DFLS_RES +
>> +				      (i * sizeof(dfl_res));
>
> Above two line can be put into one line, it's < 80 length.

Agreed.

>
>> +		if (dfl_res_off >= PCI_CFG_SPACE_EXP_SIZE) {
>> +			dev_err(&pcidev->dev, "%s offset too big for PCIe
>> config space\n",
>> +				__func__);
>> +			return -EINVAL;
>> +		}
>> +
>> +		dfl_res = GENMASK(31, 0);
>
> do we really need this?

Again, the assignment is ensuring that a failure to the 
pci_read_config_dword function is handled properly.

>
>> +		pci_read_config_dword(pcidev, dfl_res_off, &dfl_res);
>> +
>> +		dev_dbg(&pcidev->dev, "dfl_res 0x%x\n", dfl_res);
>
> Can be read by lspci even without driver, so we may not really need this
> debug information here.


I suppose the call to dev_dbg can be removed.

>
>> +
>> +		bar = dfl_res & PCI_VNDR_DFLS_RES_BAR_MASK;
>> +		if (bar >= PCI_STD_NUM_BARS) {
>> +			dev_err(&pcidev->dev, "%s bad bar number %d\n",
>> +				__func__, bar);
>> +			return -EINVAL;
>> +		}
>> +
>> +		len = pci_resource_len(pcidev, bar);
>> +		if (len == 0) {
>> +			dev_err(&pcidev->dev, "%s unmapped bar
>> number %d\n",
>> +				__func__, bar);
>> +			return -EINVAL;
>
> can be covered by below case, as mentioned in previous patch.
Agreed, I forgot to remove it.
>
>> +		}
>> +
>> +		offset = dfl_res & PCI_VNDR_DFLS_RES_OFF_MASK;
>> +		if (offset >= len) {
>> +			dev_err(&pcidev->dev, "%s bad offset %u >= %pa\n",
>> +				__func__, offset, &len);
>> +			return -EINVAL;
>> +		}
>> +
>> +		dev_dbg(&pcidev->dev, "%s BAR %d offset 0x%x\n",
>> __func__, bar, offset);
>> +
>> +		len -= offset;
>> +
>> +		start = pci_resource_start(pcidev, bar) + offset;
>> +
>> +		dfl_fpga_enum_info_add_dfl(info, start, len);
>
> That means everytime, we pass [start, endofbar] region to dfl core
> for enumeration, if there are multiple DFLs in one bar, then each range
> ends at the same endofbar, it seems fine as enumeration can be done
> one by one, but ideally the best case is that this capability can provide
> end address or size too, right? It is possible that information can be
> added to the capability as well? then we don't have such limitation.
>
> Hao

I am not sure having more than one DFL in a bar serves any purpose over a 
single DFL.  Regardless, I think the consistency of just having Offset/BIR 
in the VSEC is better than adding more infomation that has little or no added value.

>> +	}
>> +
>> +	return 0;
>> +}
>> +
>>  static int find_dfls_by_default(struct pci_dev *pcidev,
>>  				struct dfl_fpga_enum_info *info)
>>  {
>> @@ -220,7 +306,10 @@ static int cci_enumerate_feature_devs(struct
>> pci_dev *pcidev)
>>  			goto irq_free_exit;
>>  	}
>>
>> -	ret = find_dfls_by_default(pcidev, info);
>> +	ret = find_dfls_by_vsec(pcidev, info);
>> +	if (ret == -ENODEV)
>> +		ret = find_dfls_by_default(pcidev, info);
>> +
>>  	if (ret)
>>  		goto irq_free_exit;
>>
>> --
>> 2.25.2
>
>