linux-kernel - Re: [PATCH] pci: aardvark: Wait for endpoint to be ready before training link

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Message-ID: <20190424165002.GA26089@e121166-lin.cambridge.arm.com>
Date:   Wed, 24 Apr 2019 17:50:02 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Remi Pommarel <repk@...plefau.lt>
Cc:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Miquel Raynal <miquel.raynal@...tlin.com>
Subject: Re: [PATCH] pci: aardvark: Wait for endpoint to be ready before
 training link

On Wed, Apr 24, 2019 at 12:29:18AM +0200, Remi Pommarel wrote:
> Hi,
> 
> On Tue, Apr 23, 2019 at 05:32:15PM +0100, Lorenzo Pieralisi wrote:
> > On Wed, Mar 13, 2019 at 10:37:52PM +0100, Remi Pommarel wrote:
> > > When configuring pcie reset pin from gpio (e.g. initially set by
> > > u-boot) to pcie function this pin goes low for a brief moment
> > > asserting the PERST# signal. Thus connected device enters fundamental
> > > reset process and link configuration can only begin after a minimal
> > > 100ms delay (see [1]).
> > > 
> > > This makes sure that link is configured after at least 100ms from
> > > beginning of probe() callback (shortly after the reset pin function
> > > configuration switch through pinctrl subsytem).

I am a bit lost, what's the connection between the probe() callback
and the reset pin function configuration ?

Please elaborate.

> > > 
> > > [1] "PCI Express Base Specification", REV. 2.1
> > >         PCI Express, March 4 2009, 6.6.1 Conventional Reset
> > > 
> > > Signed-off-by: Remi Pommarel <repk@...plefau.lt>
> > > ---
> > >  drivers/pci/controller/pci-aardvark.c | 17 ++++++++++++++---
> > >  1 file changed, 14 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> > > index a30ae7cf8e7e..70a1023d0ef1 100644
> > > --- a/drivers/pci/controller/pci-aardvark.c
> > > +++ b/drivers/pci/controller/pci-aardvark.c
> > > @@ -177,6 +177,9 @@
> > >  
> > >  #define PIO_TIMEOUT_MS			1
> > >  
> > > +/* Endpoint can take up to 100ms to be ready after a reset */
> > > +#define ENDPOINT_RST_MS			100
> > > +
> > >  #define LINK_WAIT_MAX_RETRIES		10
> > >  #define LINK_WAIT_USLEEP_MIN		90000
> > >  #define LINK_WAIT_USLEEP_MAX		100000
> > > @@ -242,8 +245,10 @@ static int advk_pcie_wait_for_link(struct advk_pcie *pcie)
> > >  	return -ETIMEDOUT;
> > >  }
> > >  
> > > -static void advk_pcie_setup_hw(struct advk_pcie *pcie)
> > > +static void
> > > +advk_pcie_setup_hw(struct advk_pcie *pcie, unsigned long ep_rdy_time)
> > 
> > Nit: I prefer the prototype to be in one line, I wrap it for you.
> > 
> > I am wondering why you need to pass in ep_rdy_time parameter when you
> > can easily compute it in the function itself.
> > 
> 
> The only reason for that is because the sooner I get the jiffies the
> lower the delay has to be. I was trying to reduce the impact of this
> delay to a minimum, but maybe the improvement is not worth it.

That should just be (roughly) some microseconds unless there is
something I am missing. Try to measure it :)

More importantly, I would ask you to elaborate a bit more
about the logic behind this patch, see above because I need
to understand the logic behind pinctrl reset and the probe()
hook execution ordering.

Thanks,
Lorenzo