linux-kernel - Re: [PATCH v4 2/2] phy: qcom: qmp-pcie: Add PHY register retention support

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Message-ID: <20250225081744.3aprpztylrdgs2cl@thinkpad>
Date: Tue, 25 Feb 2025 13:47:44 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: "Wenbin Yao (Consultant)" <quic_wenbyao@...cinc.com>
Cc: Konrad Dybcio <konrad.dybcio@....qualcomm.com>, vkoul@...nel.org,
	kishon@...nel.org, p.zabel@...gutronix.de,
	dmitry.baryshkov@...aro.org, abel.vesa@...aro.org,
	quic_qianyu@...cinc.com, neil.armstrong@...aro.org,
	quic_devipriy@...cinc.com, linux-arm-msm@...r.kernel.org,
	linux-phy@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] phy: qcom: qmp-pcie: Add PHY register retention
 support

On Tue, Feb 25, 2025 at 04:06:16PM +0800, Wenbin Yao (Consultant) wrote:
> On 2/24/2025 8:24 PM, Manivannan Sadhasivam wrote:
> > On Mon, Feb 24, 2025 at 12:46:44PM +0100, Konrad Dybcio wrote:
> > > On 24.02.2025 9:46 AM, Wenbin Yao (Consultant) wrote:
> > > > On 2/24/2025 3:33 PM, Manivannan Sadhasivam wrote:
> > > > > On Thu, Feb 20, 2025 at 06:22:53PM +0800, Wenbin Yao wrote:
> > > > > > From: Qiang Yu <quic_qianyu@...cinc.com>
> > > > > > 
> > > > > > Some QCOM PCIe PHYs support no_csr reset. Unlike BCR reset which resets the
> > > > > > whole PHY (hardware and register), no_csr reset only resets PHY hardware
> > > > > > but retains register values, which means PHY setting can be skipped during
> > > > > > PHY init if PCIe link is enabled in booltloader and only no_csr is toggled
> > > > > > after that.
> > > > > > 
> > > > > > Hence, determine whether the PHY has been enabled in bootloader by
> > > > > > verifying QPHY_START_CTRL register. If it's programmed and no_csr reset is
> > > > > > available, skip BCR reset and PHY register setting to establish the PCIe
> > > > > > link with bootloader - programmed PHY settings.
> > > > > > 
> > > > > > Signed-off-by: Qiang Yu <quic_qianyu@...cinc.com>
> > > > > > Signed-off-by: Wenbin Yao <quic_wenbyao@...cinc.com>
> > > > > Some nitpicks below.
> > > > > 
> > > > > > ---
> > > [...]
> > > 
> > > > > > +     * In this way, no matter whether the PHY settings were initially
> > > > > > +     * programmed by bootloader or PHY driver itself, we can reuse them
> > > > > It is really possible to have bootloader not programming the init sequence for
> > > > > no_csr reset platforms? The comment sounds like it is possible. But I heard the
> > > > > opposite.
> > > > PCIe3 on X1E80100 QCP is disabled by default in UEFI. We need to enable it
> > > > manually in UEFI shell if we want.
> > > IIUC this will not be a concern going forward, and this is a special case
> > > 
> > I'm wondering how many *special* cases we may have to deal with going forward.
> > Anyhow, I would propose to atleast throw an error and fail probe() if:
> > 
> > * the platform has no_csr reset AND
> > * bootloader has not initialized the PHY AND
> > * there are no init sequences in the kernel
> > 
> > - Mani
> 
> Hmmm, regardless of whether it's a special case, we can't assume that UEFI
> will enable the PHY supporting no_csr reset on all platforms. It's a bit
> risky. If we make such an assumption, we also won't need to check whether
> the PHY is enabled by UEFI during powering on. We just need to check
> whether no_csr reset is available.
> 

I am not supportive of this assumption to be clear. While I am OK with relying
on no_csr reset and bootloader programming the PHY, we should also make sure to
catch if the PHY doesn't initialize it. Otherwise, the driver would assume that
the PHY is working, but the users won't see any PCIe devices.

> But it makes sense to check the exsitence of PHY senquence. How about
> adding the check in qmp_pcie_init, if a PHY supports no_csr reset and isn't
> initialized in UEFI and there is no cfg->tbls, return error and print some
> error log so that the PCIe controller will fail to probe.
> 

Sounds good to me.

- Mani

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