Re: [PATCH v2] PCI: pciehp: Report degraded links via link bandwidth notification

[Alex_Gagniuc]

On 2/24/19 8:29 PM, Lukas Wunner wrote:
> On Fri, Dec 07, 2018 at 12:20:00PM -0600, Alexandru Gagniuc wrote:
> 
> 
>> Q: Why is this unconditionally compiled in?
>> A: The symmetrical check in pci probe() is also always compiled in.
> 
> Hm, it looks like the convention is to provide a separate Kconfig entry
> for each port service.

Does the convention still make sense in light of the symmetry reason?

>> Q: Why call module_init() instead of adding a call in pcie_init_services() ?
>> A: A call in pcie_init_services() also requires a prototype in portdrv.h, a
>> non-static implementation in bw_notification.c. Using module_init() is
>> functionally equivalent, and takes less code.
> 
> Commit c29de84149ab ("PCI: portdrv: Initialize service drivers directly")
> moved away from module_init() on purpose, apparently to fix a race
> condition.

*GROWL*


> What if the link is retrained at the same speed/width?  Intuitively
> I'd compare the speed in the Link Status Register to what is cached
> in the cur_bus_speed member of struct pci_bus and print a message
> only if the speed has changed.  (Don't we need to cache the width as
> well?)

There are two mechanisms to bring a degraded link back to full BW.
   1. Secondary bus reset, which results in the device being tore down 
along with our cached speed value.
   2. Set the PCI_EXP_LNKCTL_LD (Link Disable) bit and clear it. We do 
that as part of the pciehp teardown path. We'd lose our cached value 
just like in the first case.

>> +static irqreturn_t pcie_bw_notification_irq(int irq, void *context)
>> [...]
> 
> You need to hold pci_bus_sem [...]
> This may sleep, so request the IRQ with request_threaded_irq() [...]

Good catch! Thanks!
All other issues you pointed out should be resolved in next version.

Alex