linux-kernel - Re: [PATCH 3/4] PCI: qcom: Read back PARF

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Message-ID: <20240102170529.GE4917@thinkpad>
Date: Tue, 2 Jan 2024 22:35:29 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Konrad Dybcio <konrad.dybcio@...aro.org>
Cc: Bjorn Andersson <andersson@...nel.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Stanimir Varbanov <svarbanov@...sol.com>,
	Andrew Murray <amurray@...goodpenguin.co.uk>,
	Vinod Koul <vkoul@...nel.org>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	linux-arm-msm@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] PCI: qcom: Read back PARF_LTSSM register

On Wed, Dec 27, 2023 at 11:17:21PM +0100, Konrad Dybcio wrote:
> To ensure write completion, read the PARF_LTSSM register after setting
> the LTSSM enable bit before polling for "link up".
> 
> Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>

I'd consider this as a bug since if the LTSSM write gets cached in Write Buffer,
then the polling time becomes wrong in dw_pcie_wait_for_link(), leading to
false link_up failure.

Although both of the write/read (LTSSM in qcom_pcie_2_3_2_ltssm_enable(), and
PCI_EXP_LNKSTA in qcom_pcie_link_up()) belong to PCIe domain, they belong to
different regions (PARF, DBI). So I'm not sure we can safely ignore the write
completion issue. So,

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>

- Mani

> ---
>  drivers/pci/controller/dwc/pcie-qcom.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
> index a02dc197c495..3d77269e70da 100644
> --- a/drivers/pci/controller/dwc/pcie-qcom.c
> +++ b/drivers/pci/controller/dwc/pcie-qcom.c
> @@ -540,6 +540,7 @@ static void qcom_pcie_2_3_2_ltssm_enable(struct qcom_pcie *pcie)
>  	val = readl(pcie->parf + PARF_LTSSM);
>  	val |= LTSSM_EN;
>  	writel(val, pcie->parf + PARF_LTSSM);
> +	readl(pcie->parf + PARF_LTSSM);
>  }
>  
>  static int qcom_pcie_get_resources_2_3_2(struct qcom_pcie *pcie)
> 
> -- 
> 2.43.0
> 

-- 
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