linux-kernel - Re: [PATCH v1 3/5] PCI: tegra: Use readl_poll

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Message-ID: <CANAwSgSeOrVjkuFbPKAPXDnCrsApcgePEs3D6MWwtsu9nYNesw@mail.gmail.com>
Date: Fri, 24 Oct 2025 11:47:02 +0530
From: Anand Moon <linux.amoon@...il.com>
To: Manivannan Sadhasivam <mani@...nel.org>
Cc: Lorenzo Pieralisi <lpieralisi@...nel.org>, Krzysztof Wilczyński <kwilczynski@...nel.org>, 
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Thierry Reding <thierry.reding@...il.com>, Jonathan Hunter <jonathanh@...dia.com>, 
	"open list:PCI NATIVE HOST BRIDGE AND ENDPOINT DRIVERS" <linux-pci@...r.kernel.org>, 
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" <devicetree@...r.kernel.org>, 
	"open list:TEGRA ARCHITECTURE SUPPORT" <linux-tegra@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>, 
	Mikko Perttunen <mperttunen@...dia.com>
Subject: Re: [PATCH v1 3/5] PCI: tegra: Use readl_poll_timeout() for link
 status polling

Hi Manivannan,

On Tue, 21 Oct 2025 at 07:13, Manivannan Sadhasivam <mani@...nel.org> wrote:
>
> On Mon, Oct 20, 2025 at 05:47:15PM +0530, Anand Moon wrote:
> > Hi Manivannan,
> >
> > Thanks for your review comment.
> >
> > On Sun, 19 Oct 2025 at 13:20, Manivannan Sadhasivam <mani@...nel.org> wrote:
> > >
> > > On Fri, Sep 26, 2025 at 12:57:44PM +0530, Anand Moon wrote:
> > > > Replace the manual `do-while` polling loops with the readl_poll_timeout()
> > > > helper when checking the link DL_UP and DL_LINK_ACTIVE status bits
> > > > during link bring-up. This simplifies the code by removing the open-coded
> > > > timeout logic in favor of the standard, more robust iopoll framework.
> > > > The change improves readability and reduces code duplication.
> > > >
> > > > Cc: Thierry Reding <thierry.reding@...il.com>
> > > > Cc: Mikko Perttunen <mperttunen@...dia.com>
> > > > Signed-off-by: Anand Moon <linux.amoon@...il.com>
> > > > ---
> > > > v1: dropped the include  <linux/iopoll.h> header file.
> > > > ---
> > > >  drivers/pci/controller/pci-tegra.c | 37 +++++++++++-------------------
> > > >  1 file changed, 14 insertions(+), 23 deletions(-)
> > > >
> > > > diff --git a/drivers/pci/controller/pci-tegra.c b/drivers/pci/controller/pci-tegra.c
> > > > index 07a61d902eae..b0056818a203 100644
> > > > --- a/drivers/pci/controller/pci-tegra.c
> > > > +++ b/drivers/pci/controller/pci-tegra.c
> > > > @@ -2169,37 +2169,28 @@ static bool tegra_pcie_port_check_link(struct tegra_pcie_port *port)
> > > >       value |= RP_PRIV_MISC_PRSNT_MAP_EP_PRSNT;
> > > >       writel(value, port->base + RP_PRIV_MISC);
> > > >
> > > > -     do {
> > > > -             unsigned int timeout = TEGRA_PCIE_LINKUP_TIMEOUT;
> > > > +     while (retries--) {
> > > > +             int err;
> > > >
> > > > -             do {
> > > > -                     value = readl(port->base + RP_VEND_XP);
> > > > -
> > > > -                     if (value & RP_VEND_XP_DL_UP)
> > > > -                             break;
> > > > -
> > > > -                     usleep_range(1000, 2000);
> > > > -             } while (--timeout);
> > > > -
> > > > -             if (!timeout) {
> > > > +             err = readl_poll_timeout(port->base + RP_VEND_XP, value,
> > > > +                                      value & RP_VEND_XP_DL_UP,
> > > > +                                      1000,
> > >
> > > The delay between the iterations had range of (1000, 2000), now it will become
> > > (250, 1000). How can you ensure that this delay is sufficient?
> > >
> > I asked if the timeout should be increased for the loops, but Mikko
> > Perttunen said that 200ms delay is fine.
> >
>
> readl_poll_timeout() internally uses usleep_range(), which transforms the 1000us
> delay into, usleep_range(251, 1000). So the delay *could* theoretically be 251us
> * 200 = ~50ms.
>
> So I doubt it will be sifficient, as from the old code, it looks like the
> hardware could take around 200ms to complete link up.
>
Instead of implementing a busy-waiting while loop with udelay, a more
efficient and responsive approach is to use the readl_poll_timeout()
function. This function periodically polls a memory-mapped address, waiting
for a condition to be met or for a specified timeout to occur.

If there are any issues with HW, we could extend the timeout to compensate.
> - Mani
Thanks
-Anand
>
> --
> மணிவண்ணன் சதாசிவம்