Re: [PATCH V6 0/5] PCIe TPH and cache direct injection support

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Sep 27, 2024 at 04:56:48PM -0500, Wei Huang wrote:
> Hi All,
> 
> TPH (TLP Processing Hints) is a PCIe feature that allows endpoint
> devices to provide optimization hints for requests that target memory
> space. These hints, in a format called steering tag (ST), are provided
> in the requester's TLP headers and allow the system hardware, including
> the Root Complex, to optimize the utilization of platform resources
> for the requests.
> 
> Upcoming AMD hardware implement a new Cache Injection feature that
> leverages TPH. Cache Injection allows PCIe endpoints to inject I/O
> Coherent DMA writes directly into an L2 within the CCX (core complex)
> closest to the CPU core that will consume it. This technology is aimed
> at applications requiring high performance and low latency, such as
> networking and storage applications.
> 
> This series introduces generic TPH support in Linux, allowing STs to be
> retrieved and used by PCIe endpoint drivers as needed. As a
> demonstration, it includes an example usage in the Broadcom BNXT driver.
> When running on Broadcom NICs with the appropriate firmware, it shows
> substantial memory bandwidth savings and better network bandwidth using
> real-world benchmarks. This solution is vendor-neutral and implemented
> based on industry standards (PCIe Spec and PCI FW Spec).
> 
> V5->V6:
>  * Rebase on top of pci/main (tag: pci-v6.12-changes)
>  * Fix spellings and FIELD_PREP/bnxt.c compilation errors (Simon)
>  * Move tph.c to drivers/pci directory (Lukas)
>  * Remove CONFIG_ACPI dependency (Lukas)
>  * Slightly re-arrange save/restore sequence (Lukas)

Thanks, I'll wait for the kernel test robot warnings to be resolved.

In patch 2/5, reword commit logs as imperative mood, e.g.,
s/X() is added/Add X()/, as you've already done for 1/5 and 3/5.

Maybe specify the ACPI _DSM name?  This would help users know whether
their system can use this, or help them request that a vendor
implement the _DSM.

In patch 4/5, s/sustancial/substantial/.  I guess the firmware you
refer to here means the system firmware that would provide the _DSM
required for this to work, i.e., not firmware on the NIC itself?
Would be helpful for users to have a hint as to how to tell whether to
expect a benefit on their system.

The 5/5 commit log could say what the patch *does*, not what *could*
be done (the subject does say what the patch does, but it's nice if
it's in the commit log as well so it's complete by itself).  Also, a
hint that using the steering tag helps direct DMA writes to a cache
close to the CPU expected to consume it might be helpful to motivate
the patch.

Bjorn