Re: [PATCH v5] PCI: dwc: Wait for link up only if link is started

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

On Tue, Jan 30, 2024 at 02:30:27PM +0530, Ajay Agarwal wrote:

[...]

> > > > > > If that's the case with your driver, when are you starting the link training?
> > > > > > 
> > > > > The link training starts later based on a userspace/debugfs trigger.
> > > > > 
> > > > 
> > > > Why does it happen as such? What's the problem in starting the link during
> > > > probe? Keep it in mind that if you rely on the userspace for starting the link
> > > > based on a platform (like Android), then if the same SoC or peripheral instance
> > > > get reused in other platform (non-android), the it won't be a seamless user
> > > > experience.
> > > > 
> > > > If there are any other usecases, please state them.
> > > > 
> > > > - Mani
> > > >
> > > This SoC is targeted for an android phone usecase and the endpoints
> > > being enumerated need to go through an appropriate and device specific
> > > power sequence which gets triggered only when the userspace is up. The
> > > PCIe probe cannot assume that the EPs have been powered up already and
> > > hence the link-up is not attempted.
> > 
> > Still, I do not see the necessity to not call start_link() during probe. If you
> I am not adding any logic/condition around calling the start_link()
> itself. I am only avoiding the wait for the link to be up if the
> controller driver has not defined start_link().
> 

I'm saying that not defining the start_link() callback itself is wrong.

> > add PROBE_PREFER_ASYNCHRONOUS to your controller driver, this delay would become
> > negligible. The reason why I'm against not calling start_link() is due to below
> > reasons:
> > 
> > 1. If the same SoC gets reused for other platforms, even other android phones
> > that powers up the endpoints during boot, then it creates a dependency with
> > userspace to always start the link even though the devices were available.
> > That's why we should never fix the behavior of the controller drivers based on a
> > single platform.
> I wonder how the behavior is changing with this patch. Do you have an
> example of a platform which does not have start_link() defined but would
> like to still wait for a second for the link to come up?
> 

Did you went through my reply completely? I mentioned that the 1s delay would be
gone if you add the async flag to your driver and you are ignoring that.

But again, I'm saying that not defining start_link() itself is wrong and I've
already mentioned the reasons.

> For example, consider the intel-gw driver. The 1 sec wait time in its
> probe path is also a waste because it explicitly starts link training
> later in time.
> 

I previously mentioned that the intel-gw needs fixing since there is no point in
starting the link and waiting for it to come up in its probe() if the DWC core
is already doing that.

- Mani

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