linux-kernel - Re: [PATCH] PCI: dwc: Use the correct sleep function in wait_for

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Message-ID: <20240215170258.GA1292702@bhelgaas>
Date: Thu, 15 Feb 2024 11:02:58 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Konrad Dybcio <konrad.dybcio@...aro.org>,
	Jingoo Han <jingoohan1@...il.com>,
	Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
	Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	Johan Hovold <johan+linaro@...nel.org>
Subject: Re: [PATCH] PCI: dwc: Use the correct sleep function in wait_for_link

On Thu, Feb 15, 2024 at 02:35:13PM +0100, Alexander Lobakin wrote:
> From: Konrad Dybcio <konrad.dybcio@...aro.org>
> Date: Thu, 15 Feb 2024 11:39:31 +0100
> 
> > According to [1], msleep should be used for large sleeps, such as the
> > 100-ish ms one in this function. Comply with the guide and use it.
> > 
> > [1] https://www.kernel.org/doc/Documentation/timers/timers-howto.txt
> > 
> > Signed-off-by: Konrad Dybcio <konrad.dybcio@...aro.org>
> > ---
> > Tested on Qualcomm SC8280XP CRD
> > ---
> >  drivers/pci/controller/dwc/pcie-designware.c | 2 +-
> >  drivers/pci/controller/dwc/pcie-designware.h | 3 +--
> >  2 files changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
> > index 250cf7f40b85..abce6afceb91 100644
> > --- a/drivers/pci/controller/dwc/pcie-designware.c
> > +++ b/drivers/pci/controller/dwc/pcie-designware.c
> > @@ -655,7 +655,7 @@ int dw_pcie_wait_for_link(struct dw_pcie *pci)
> >  		if (dw_pcie_link_up(pci))
> >  			break;
> >  
> > -		usleep_range(LINK_WAIT_USLEEP_MIN, LINK_WAIT_USLEEP_MAX);
> > +		msleep(LINK_WAIT_MSLEEP_MAX);
> 
> Just use fsleep(LINK_WAIT_USLEEP_MAX) and let the kernel decide which
> function to pick.

Odd.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/timers/timers-howto.rst?id=v6.7#n114
mentions fsleep() (with no real guidance about when to use it), but
https://www.kernel.org/doc/Documentation/timers/timers-howto.txt 
seems to be a stale copy from 2017, before fsleep() was added.  I
emailed helpdesk@...nel.org to see if the stale content can be
removed.

I do think fsleep() should be more widely used.

> >  /* Parameters for the waiting for link up routine */
> >  #define LINK_WAIT_MAX_RETRIES		10
> > -#define LINK_WAIT_USLEEP_MIN		90000
> > -#define LINK_WAIT_USLEEP_MAX		100000
> > +#define LINK_WAIT_MSLEEP_MAX		100

Since you're touching this anyway, it would be helpful to include the
units on the timeout.

USLEEP/MSLEEP is definitely a hint, but I think the "SLEEP" part
suggests something about atomic/non-atomic context and isn't relevant
to the time interval itself, and something like "TIMEOUT" would be
better.

I think an explicit "_US" or "_MS" would better indicate the units.

This is turning into a long tangent, but I'm not a huge fan of the
LINK_WAIT_* pattern where I have to look up the code that uses
LINK_WAIT_MAX_RETRIES and LINK_WAIT_USLEEP_MAX and do the math to see
what the actual timeout is.  Obviously not fodder for *this* patch.

Bjorn