linux-kernel - Re: [PATCH 1/5] PCI/switchtec: Error out MRPC execution when no GAS access

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Message-ID: <37d0615c8a9a5e7c55527f4d478a0e707292c1ec.camel@microchip.com>
Date:   Fri, 8 Oct 2021 00:06:18 +0000
From:   <Kelvin.Cao@...rochip.com>
To:     <helgaas@...nel.org>
CC:     <kurt.schwemmer@...rosemi.com>, <bhelgaas@...gle.com>,
        <linux-pci@...r.kernel.org>, <kelvincao@...look.com>,
        <logang@...tatee.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] PCI/switchtec: Error out MRPC execution when no GAS
 access

On Thu, 2021-10-07 at 16:23 -0500, Bjorn Helgaas wrote:
> On Wed, Oct 06, 2021 at 09:27:49PM +0000, Kelvin.Cao@...rochip.com
> wrote:
> > On Wed, 2021-10-06 at 15:20 -0500, Bjorn Helgaas wrote:
> > > On Wed, Oct 06, 2021 at 07:00:55PM +0000, 
> > > Kelvin.Cao@...rochip.com
> > > wrote:
> > > So wait, you mean you just intentionally ask the firmware to
> > > reset, knowing that the device will be unusable until the user
> > > reboots or does a manual rescan?  And the way to improve this is
> > > for the driver to report an error to the user instead of hanging?
> > > I *guess* that might be some sort of improvement, but seems like
> > > a
> > > pretty small one.
> > 
> > Yes, however, I believe it's something our users really like to
> > have...  With this, they can do their user space
> > programming/scripting more easily in a synchronous fashion.
> > 
> > > >   - The firwmare crashes and doesn't respond, which normally is
> > > >   the reason for users to issue a firmware reset command to try
> > > >   to recover it via either the driver or a sideband interface.
> > > >   The firmware may not be able to recover by a reset in some
> > > >   extream situations like hardware errors, so that an error
> > > >   return is probably all the users can get before another level
> > > >   of recovery happens.
> > > > 
> > > > So I'd think this patch is still making the driver better in
> > > > some way.
> 
> OK.  I still think the fact that all these different mechanisms can
> reset the device behind your back and make the switch and anything on
> the other side of it just stop working is ..., well, let's just say
> it's quite surprising to me.

Actually there're mechanisms like permission control to limit what
people can do in the firmware, so I guess it's not as bad as it sounds
like. 
> 
> Well, at least this isn't quite so much a mystery anymore and maybe
> we
> can improve the commit log.  E.g., maybe something like this:
> 
>   A firmware hard reset may be initiated by various mechanisms
>   including a UART interface, TWI sideband interface from BMC, MRPC
>   command from userspace, etc.  The switchtec management driver is
>   unaware of these resets.
> 
>   The reset clears PCI state including the BARs and Memory Space
>   Enable bits, so the device no longer responds to the MMIO accesses
>   the driver uses to operate it.
> 
>   MMIO reads to the device will fail with a PCIe error.  When the
> root
>   complex handles that error, it typically fabricates ~0 data to
>   complete the CPU read.
> 
>   Check for this sort of error by reading the device ID from MMIO
>   space.  This ID can never be ~0, so if we see that value, it
>   probably means the PCIe Memory Read failed and we should return an
>   error indication to the application using the switchtec driver.

It looks good to me, the commit log removes the ambiguity. Let me know
if you prefer a v2 patchset with the updated commit log and naming
issue fix.

Thank you Bjorn for your patience and time! 

Kelvin