linux-kernel - Re: [PATCH v3 3/3] vfio/nvgrace-gpu: Check the HBM training and C2C link status

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Message-ID:
 <SA1PR12MB7199624F639518D3CA59C7F4B01B2@SA1PR12MB7199.namprd12.prod.outlook.com>
Date: Fri, 17 Jan 2025 21:13:52 +0000
From: Ankit Agrawal <ankita@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>
CC: Jason Gunthorpe <jgg@...dia.com>, Yishai Hadas <yishaih@...dia.com>,
	"shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.com>, "kevin.tian@...el.com"
	<kevin.tian@...el.com>, Zhi Wang <zhiw@...dia.com>, Aniket Agashe
	<aniketa@...dia.com>, Neo Jia <cjia@...dia.com>, Kirti Wankhede
	<kwankhede@...dia.com>, "Tarun Gupta (SW-GPU)" <targupta@...dia.com>, Vikram
 Sethi <vsethi@...dia.com>, Andy Currid <acurrid@...dia.com>, Alistair Popple
	<apopple@...dia.com>, John Hubbard <jhubbard@...dia.com>, Dan Williams
	<danw@...dia.com>, "Anuj Aggarwal (SW-GPU)" <anuaggarwal@...dia.com>, Matt
 Ochs <mochs@...dia.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/3] vfio/nvgrace-gpu: Check the HBM training and C2C
 link status


>> > We're accessing device memory here but afaict the memory enable bit of
>> > the command register is in an indeterminate state.  What happens if you
>> > use setpci to clear the memory enable bit or 'echo 0 > enable' before
>> > binding the driver?  Thanks,
>> >
>> > Alex
>>
>> Hi Alex, sorry I didn't understand how we are accessing device memory here if
>> the C2C_LINK_BAR0_OFFSET and HBM_TRAINING_BAR0_OFFSET are BAR0 regs.
>> But anyways, I tried 'echo 0 > <sysfs_path>/enable' before device bind. I am not
>> observing any issue and the bind goes through.
>>
>> Or am I missing something?
>
> BAR0 is what I'm referring to as device memory.  We cannot access
> registers in BAR0 unless the memory space enable bit of the command
> register is set.  The nvgrace-gpu driver makes no effort to enable this
> and I don't think the PCI core does before probe either.  Disabling
> through sysfs will only disable if it was previously enabled, so
> possibly that test was invalid.  Please try with setpci:
>
> # Read command register
> $ setpci -s xxxx:xx:xx.x COMMAND
> # Clear memory enable
> $ setpci -s xxxx:xx:xx.x COMMAND=0:2
> # Re-read command register
> $ setpci -s xxxx:xx:xx.x COMMAND
>
> Probe driver here now that the memory enable bit should re--back as
> unset.  Thanks,
>
> Alex

Ok, yeah. I tried to disable through setpci, and the probe is failing with ETIME.
Should we check if disabled and return -EIO for such situation to differentiate
from timeout?