linux-kernel - Re: [PATCH v8 5/7] PCI: Store # of supported End-End TLP Prefixes

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Message-ID: <20250108205613.GB1342186@yaz-khff2.amd.com>
Date: Wed, 8 Jan 2025 15:56:13 -0500
From: Yazen Ghannam <yazen.ghannam@....com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	Krzysztof Wilczyński <kw@...ux.com>,
	Lukas Wunner <lukas@...ner.de>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	linux-kernel@...r.kernel.org,
	Mahesh J Salgaonkar <mahesh@...ux.ibm.com>,
	Oliver O'Halloran <oohall@...il.com>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v8 5/7] PCI: Store # of supported End-End TLP Prefixes

On Wed, Dec 18, 2024 at 04:37:45PM +0200, Ilpo Järvinen wrote:
> eetlp_prefix_path in the struct pci_dev tells if End-End TLP Prefixes
> are supported by the path or not, the value is only calculated if
> CONFIG_PCI_PASID is set.
> 
> The Max End-End TLP Prefixes field in the Device Capabilities Register
> 2 also tells how many (1-4) End-End TLP Prefixes are supported (PCIe
> r6.2 sec 7.5.3.15). The number of supported End-End Prefixes is useful
> for reading correct number of DWORDs from TLP Prefix Log register in AER
> capability (PCIe r6.2 sec 7.8.4.12).
> 
> Replace eetlp_prefix_path with eetlp_prefix_max and determine the
> number of supported End-End Prefixes regardless of CONFIG_PCI_PASID so
> that an upcoming commit generalizing TLP Prefix Log register reading
> does not have to read extra DWORDs for End-End Prefixes that never will
> be there.
> 
> The value stored into eetlp_prefix_max is directly derived from
> device's Max End-End TLP Prefixes and does not consider limitations
> imposed by bridges or the Root Port beyond supported/not supported
> flags. This is intentional for two reasons:
> 
>   1) PCIe r6.2 spec sections r6.1 2.2.10.4 & 6.2.4.4 indicate that TLP
>   is handled malformed only if the number of prefixes exceed the number
>   of Max End-End TLP Prefixes, which seems to be the case even if the
>   device could never receive that many prefixes due to smaller maximum
>   imposed by a bridge or the Root Port. If TLP parsing is later added,
>   this distinction is significant in interpreting what is logged by the
>   TLP Prefix Log registers and the value matching to the Malformed TLP
>   threshold is going to be more useful.
> 
>   2) TLP Prefix handling happens autonomously on a low layer and the
>   value in eetlp_prefix_max is not programmed anywhere by the kernel
>   (i.e., there is no limiter OS can control to prevent sending more
>   than n TLP Prefixes).
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>

Reviewed-by: Yazen Ghannam <yazen.ghannam@....com>

Thanks,
Yazen