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

From: Bjorn Helgaas <helgaas@kernel.org>
To: Dongdong Liu <liudongdong3@huawei.com>
Cc: hch@infradead.org, kw@linux.com, logang@deltatee.com,
	leon@kernel.org, linux-pci@vger.kernel.org, rajur@chelsio.com,
	hverkuil-cisco@xs4all.nl, linux-media@vger.kernel.org,
	netdev@vger.kernel.org
Subject: Re: [PATCH V10 4/8] PCI/sysfs: Add a 10-Bit Tag sysfs file PCIe Endpoint devices
Date: Wed, 27 Oct 2021 17:28:39 -0500	[thread overview]
Message-ID: <20211027222839.GA252933@bhelgaas> (raw)
In-Reply-To: <20211009104938.48225-5-liudongdong3@huawei.com>

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

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

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