Re: [PATCH] pci-driver: Add driver load messages

[Prarit Bhargava <prarit@redhat.com>]

On 1/26/21 8:14 AM, Leon Romanovsky wrote:
> On Tue, Jan 26, 2021 at 07:54:46AM -0500, Prarit Bhargava wrote:
>>   Leon Romanovsky <leon@kernel.org> wrote:
>>> On Mon, Jan 25, 2021 at 02:41:38PM -0500, Prarit Bhargava wrote:
>>>> There are two situations where driver load messages are helpful.
>>>>
>>>> 1) Some drivers silently load on devices and debugging driver or system
>>>> failures in these cases is difficult.  While some drivers (networking
>>>> for example) may not completely initialize when the PCI driver probe() function
>>>> has returned, it is still useful to have some idea of driver completion.
>>>
>>> Sorry, probably it is me, but I don't understand this use case.
>>> Are you adding global to whole kernel command line boot argument to debug
>>> what and when?
>>>
>>> During boot:
>>> If device success, you will see it in /sys/bus/pci/[drivers|devices]/*.
>>> If device fails, you should get an error from that device (fix the
>>> device to return an error), or something immediately won't work and
>>> you won't see it in sysfs.
>>>
>>
>> What if there is a panic during boot?  There's no way to get to sysfs.
>> That's the case where this is helpful.
> 
> How? If you have kernel panic, it means you have much more worse problem
> than not-supported device. If kernel panic was caused by the driver, you
> will see call trace related to it. If kernel panic was caused by
> something else, supported/not supported won't help here.

I still have no idea *WHICH* device it was that the panic occurred on.
> 
>>
>>> During run:
>>> We have many other solutions to get debug prints during run, for example
>>> tracing, which is possible to toggle dynamically.
>>>
>>> Right now, my laptop will print 34 prints on boot and endless amount during
>>> day-to-day usage.
>>>
>>> ➜  kernel git:(rdma-next) ✗ lspci |wc -l
>>> 34
>>>
>>>>
>>>> 2) Storage and Network device vendors have relatively short lives for
>>>> some of their hardware.  Some devices may continue to function but are
>>>> problematic due to out-of-date firmware or other issues.  Maintaining
>>>> a database of the hardware is out-of-the-question in the kernel as it would
>>>> require constant updating.  Outputting a message in the log would allow
>>>> different OSes to determine if the problem hardware was truly supported or not.
>>>
>>> And rely on some dmesg output as a true source of supported/not supported and
>>> making this ABI which needs knob in command line. ?
>>
>> Yes.  The console log being saved would work as a true source of load
>> messages to be interpreted by an OS tool.  But I see your point about the
>> knob below...
> 
> You will need much more stronger claim than the above if you want to proceed
> ABI path through dmesg prints.
> 

See my answer below.  I agree with you on the ABI statement.

>>
>>>
>>>>
>>>> Add optional driver load messages from the PCI core that indicates which
>>>> driver was loaded, on which slot, and on which device.
>>>
>>> Why don't you add simple pr_debug(..) without any knob? You will be able
>>> to enable/disable it through dynamic prints facility.
>>
>> Good point.  I'll wait for more feedback and submit a v2 with pr_debug.
> 
> Just to be clear, none of this can be ABI and any kernel print can
> be changed or removed any minute without any announcement.

Yes, that's absolutely the case and I agree with you that nothing can guarantee
ABI of those pr_debug() statements.  They are *debug* after all.

P.

> 
> Thanks
> 
>>
>> P.
>>
>>>
>>> Thanks
>>
>