Re: [PATCH V10 4/8] PCI/sysfs: Add a 10-Bit Tag sysfs file PCIe Endpoint devices

Hi Bjorn

Many thanks for your review.
On 2021/10/28 6:28, Bjorn Helgaas wrote:

On Sat, Oct 09, 2021 at 06:49:34PM +0800, Dongdong Liu wrote:

quoted

PCIe spec 5.0 r1.0 section 2.2.6.2 says:

  If an Endpoint supports sending Requests to other Endpoints (as
  opposed to host memory), the Endpoint must not send 10-Bit Tag
  Requests to another given Endpoint unless an implementation-specific
  mechanism determines that the Endpoint supports 10-Bit Tag Completer
  capability.

Add a 10bit_tag sysfs file, write 0 to disable 10-Bit Tag Requester
when the driver does not bind the device. The typical use case is for
p2pdma when the peer device does not support 10-Bit Tag Completer.
Write 1 to enable 10-Bit Tag Requester when RC supports 10-Bit Tag
Completer capability. The typical use case is for host memory targeted
by DMA Requests. The 10bit_tag file content indicate current status of
10-Bit Tag Requester Enable.

Don't we have a hole here?  We're adding knobs to control 10-Bit Tag
usage, but don't we have basically the same issues with Extended
(8-bit) Tags?

All PCIe completers are required to support 8-bit tags
from the "[PATCH] PCI: enable extended tags support for PCIe endpoints"
(https://patchwork.kernel.org/project/linux-arm-msm/patch/1474769434-5756-1-git-send-email-okaya@codeaurora.org/).

I ask hardware colleagues, also says all PCIe devices should support
8-bit tags completer default, so seems no need to do this for 8-bit tags.

I wonder if we should be adding a more general "tags" file that can
manage both 8-bit and 10-bit tag usage.

quoted

+static struct device_attribute dev_attr_10bit_tag = __ATTR(10bit_tag, 0644,
+							   pci_10bit_tag_show,
+							   pci_10bit_tag_store);

I think this should use DEVICE_ATTR().

Yes, will do.

Thanks,
Dongdong

Or even better, if the name doesn't start with a digit, DEVICE_ATTR_RW().
.