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

[Ajay Agarwal <ajayagarwal@google.com>]

On Wed, Jan 31, 2024 at 12:06:26AM +0530, Manivannan Sadhasivam wrote:
> On Tue, Jan 30, 2024 at 10:48:59PM +0530, Ajay Agarwal wrote:
> > On Tue, Jan 30, 2024 at 05:59:06PM +0530, Manivannan Sadhasivam wrote:
> > > 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.
> > > 
> > Whether the start_link() should be defined or not, is a different
> > design discussion. We currently have 2 drivers in upstream (intel-gw and
> > dw-plat) which do not have start_link() defined. Waiting for the link to
> > come up for the platforms using those drivers is not a good idea. And
> > that is what we are trying to avoid.
> > 
> 
> NO. The sole intention of this patch is to fix the delay observed with _your_
> out-of-tree controller driver as you explicitly said before. Impact for the
> existing 2 drivers are just a side effect.
>
Hi Mani,
What is the expectation from the pcie-designware-host driver? If the
.start_link() has to be defined by the vendor driver, then shouldn't the
probe be failed if the vendor has not defined it? Thereby failing the
probe for intel-gw and pcie-dw-plat drivers?

Additionally, if the link fails to come up even after 1 sec of wait
time, shouldn't the probe be failed in that case too?

My understanding of these drivers is that the .start_link() is an
OPTIONAL callback and that the dw_pcie_host_init should help setup the
SW structures regardless of whether the .start_link() has been defined
or not, and whether the link is up or not. The vendor should be allowed
to train the link at a later point of time as well.

Please let me know your thoughts.
> > > > > 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.
> > > 
The async probe might not help in all the cases. Consider a situation
where the PCIe is probed after the boot is already completed. The user
will face the delay then. Do you agree?

> > Yes, I did go through your suggestion of async probe and that might
> > solve my problem of the 1 sec delay. But I would like to fix the problem
> > at the core.
> > 
> 
> There is no problem at the core. The problem is with some controller drivers.
> Please do not try to fix a problem which is not there. There are no _special_
> reasons for those 2 drivers to not define start_link() callback. I'm trying to
> point you in the right path, but you are always chosing the other one.
> 
> > > 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
> > > 
> > > -- 
> > > மணிவண்ணன் சதாசிவம்
> > I think we are at a dead-end in terms of agreeing to a policy. I would
> > like the maintainers to pitch in here with their views.
> 
> I'm the maintainer of the DWC drivers that you are proposing the patch for. If
> you happen to spin future revision of this series, please carry:
> 
> Nacked-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> 
> - Mani
> 
> -- 
> மணிவண்ணன் சதாசிவம்