Re: [PATCH] PCI: mediatek-gen3: fix PCIe #PERST being de-asserted too early

From: Bjorn Helgaas <helgaas@kernel.org>
To: Daniel Golle <daniel@makrotopia.org>
Cc: linux-pci@vger.kernel.org, linux-mediatek@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	"AngeloGioacchino Del Regno"
	<angelogioacchino.delregno@collabora.com>,
	"Matthias Brugger" <matthias.bgg@gmail.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	"Lorenzo Pieralisi" <lpieralisi@kernel.org>,
	"Jianjun Wang" <jianjun.wang@mediatek.com>,
	"Ryder Lee" <ryder.lee@mediatek.com>,
	"Christian Marangi" <ansuelsmth@gmail.com>,
	"Frank Wunderlich" <linux@fw-web.de>,
	"John Crispin" <john@phrozen.org>
Subject: Re: [PATCH] PCI: mediatek-gen3: fix PCIe #PERST being de-asserted too early
Date: Tue, 10 Oct 2023 11:04:23 -0500	[thread overview]
Message-ID: <20231010160423.GA977719@bhelgaas> (raw)
In-Reply-To: <ZR-7Nm2c5s4kuOp0@pidgin.makrotopia.org>

On Fri, Oct 06, 2023 at 09:45:58AM +0200, Daniel Golle wrote:
> The driver for MediaTek gen3 PCIe hosts de-asserts all reset
> signals at the same time using a single register write operation.
> Delay the de-assertion of the #PERST signal by 100ms as required by
> PCIe CEM clause 2.2, some PCIe devices fail to come up otherwise.
> 
> Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> ---
>  drivers/pci/controller/pcie-mediatek-gen3.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/pcie-mediatek-gen3.c b/drivers/pci/controller/pcie-mediatek-gen3.c
> index e0e27645fdf4..ba8cfce03aad 100644
> --- a/drivers/pci/controller/pcie-mediatek-gen3.c
> +++ b/drivers/pci/controller/pcie-mediatek-gen3.c
> @@ -350,7 +350,13 @@ static int mtk_pcie_startup_port(struct mtk_gen3_pcie *pcie)

I feel like I'm missing something because this patch seems to be
adding a delay for T_PVPERL, but the comment before the existing
msleep() claims *it* is the T_PVPERL delay:

         * Described in PCIe CEM specification sections 2.2 (PERST# Signal)
         * and 2.2.1 (Initial Power-Up (G3 to S0)).
         * The deassertion of PERST# should be delayed 100ms (TPVPERL)
         * for the power and clock to become stable.

>  	msleep(100);
>  
>  	/* De-assert reset signals */
> -	val &= ~(PCIE_MAC_RSTB | PCIE_PHY_RSTB | PCIE_BRG_RSTB | PCIE_PE_RSTB);
> +	val &= ~(PCIE_MAC_RSTB | PCIE_PHY_RSTB | PCIE_BRG_RSTB);
> +	writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
> +
> +	msleep(100);

So I'm confused about these two sleeps.  Are they for different
parameters?

T_PVPERL is defined from "Power stable to PERST# inactive".  Do we
have any actual indication of when to start that delay, i.e., do we
have a clue about when power became stable?

> +	/* De-assert PERST# signals */
> +	val &= ~(PCIE_PE_RSTB);
>  	writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
>  
>  	/* Check if the link is up or not */
> -- 
> 2.42.0
> 
> 
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