Re: [PATCH 0/3] PCI: dwc: Cache PCIe capability offset and simplify drivers

From: Manivannan Sadhasivam <mani@kernel.org>
To: Hans Zhang <18255117159@163.com>
Cc: bhelgaas@google.com, lpieralisi@kernel.org,
	kwilczynski@kernel.org,  jingoohan1@gmail.com,
	mx@lists.linux.dev, linuxppc-dev@lists.ozlabs.org,
	 linux-amlogic@lists.infradead.org,
	linux-rockchip@lists.infradead.org,
	 linux-arm-msm@vger.kernel.org, sophgo@lists.linux.dev,
	linux-riscv@lists.infradead.org,  spacemit@lists.linux.dev,
	linux-tegra@vger.kernel.org, linux-pci@vger.kernel.org,
	 linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] PCI: dwc: Cache PCIe capability offset and simplify drivers
Date: Tue, 19 May 2026 21:45:51 +0530	[thread overview]
Message-ID: <bejh7em2a67a6wohtakovbg6wqwhjoxkuqtdompsexfm5bbzrf@yqwcqeg6yugq> (raw)
In-Reply-To: <5cc6fbcc-98eb-4da5-b123-2c04c4d39326@163.com>

On Wed, May 20, 2026 at 12:09:28AM +0800, Hans Zhang wrote:
> 
> 
> On 5/19/26 21:57, Manivannan Sadhasivam wrote:
> > On Sat, May 09, 2026 at 09:51:49PM +0800, Hans Zhang wrote:
> > > The DWC PCIe core and its many platform drivers repeatedly call
> > > dw_pcie_find_capability(pci, PCI_CAP_ID_EXP) to obtain the offset of the
> > > PCI Express Capability structure. This is wasteful and makes the code
> > > verbose. And some even search for the PCI_CAP_ID_EXP offset value within
> > > the suspend/resume functions.
> > > 
> > 
> > Sashiko has flagged some real issues with this series in accessing DBI space
> > very early and 'pci->pcie_cap' being 0.
> 
> 
> Hi Mani,
> 
> We have discussed this issue in the Cadence driver. I think it won't cause
> any problems. Specifically as follows:
> 
> https://lore.kernel.org/linux-pci/5823faec-d972-4c77-90ec-a215c686e0a8@163.com/
> """
> As per PCIe r7.0, sec 7.5.1.1.11, Since all PCI Express Functions are
> required to implement the PCI Express Capability structure, which
> must be included somewhere in this linked list.
> """
> 
> 
> 
> Bjorn also responded as follows:
> https://lore.kernel.org/linux-pci/20260505212306.GA744158@bhelgaas/
> """
> It's true that all Root Ports must have a PCIe Capability.
> """
> 

Ok, what about reading the DBI registers very early?

- Mani

-- 
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