Re: [PATCH v2 10/10] PCI: qcom: Implement shutdown() callback to properly reset the endpoint devices

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Apr 03, 2024 at 10:03:26PM +0200, Niklas Cassel wrote:
> On Wed, Apr 03, 2024 at 07:02:17PM +0530, Manivannan Sadhasivam wrote:
> > On Tue, Apr 02, 2024 at 01:18:54PM +0200, Niklas Cassel wrote:
> > > On Mon, Apr 01, 2024 at 09:20:36PM +0530, Manivannan Sadhasivam wrote:
> > > > PCIe host controller drivers are supposed to properly reset the endpoint
> > > > devices during host shutdown/reboot. Currently, Qcom driver doesn't do
> > > > anything during host shutdown/reboot, resulting in both PERST# and refclk
> > > > getting disabled at the same time. This prevents the endpoint device
> > > > firmware to properly reset the state machine. Because, if the refclk is
> > > > cutoff immediately along with PERST#, access to device specific registers
> > > > within the endpoint will result in a firmware crash.
> > > > 
> > > > To address this issue, let's call qcom_pcie_host_deinit() inside the
> > > > shutdown callback, that asserts PERST# and then cuts off the refclk with a
> > > > delay of 1ms, thus allowing the endpoint device firmware to properly
> > > > cleanup the state machine.
> ...

> For a real PCIe card, if you assert + msleep(100) + deassert PERST, surely
> the endpoint is supposed to be in a good/well defined state, regardless if
> he REFCLK was cutoff at the exact time as PERST was asserted or not?

I think this is the key point.  This PERST#/REFCLK timing requirement
seems like a defect in the endpoint.  I know there's firmware involved
and whatnot, but IMO it's the endpoint's problem to handle these issues
internally.

Bjorn