linux-kernel - Re: [PATCH v3 09/11] serial: 8250_lpss: move Quark code from PCI driver

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Message-ID: <1462359696.27858.184.camel@nexus-software.ie>
Date:	Wed, 04 May 2016 12:01:36 +0100
From:	Bryan O'Donoghue <pure.logic@...us-software.ie>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Peter Hurley <peter@...leysoftware.com>,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	Vinod Koul <vinod.koul@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	dmaengine <dmaengine@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Puustinen, Ismo" <ismo.puustinen@...el.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH v3 09/11] serial: 8250_lpss: move Quark code from PCI
 driver

On Wed, 2016-05-04 at 13:03 +0300, Andy Shevchenko wrote:
> On Wed, May 4, 2016 at 12:51 PM, Bryan O'Donoghue
> <pure.logic@...us-software.ie> wrote:
> > On Wed, 2016-05-04 at 12:42 +0300, Andy Shevchenko wrote:
> > > On Wed, May 4, 2016 at 12:31 PM, Bryan O'Donoghue
> > > <pure.logic@...us-software.ie> wrote:
> > > > Andy,
> > > > 
> > > > If you are going to start removing working PCI devices from the
> > > > PCI
> > > > config table in favour of a shim in SERIAL_8250_LPSS then the
> > > > very
> > > > minimum should be some sort of dependency link between
> > > > SERIAL_8250_LPSS
> > > > and CONFIG_SERIAL_8250_PCI in kconfig.
> > > > 
> > > > A user could reasonably read the QRK datasheet - switch on
> > > > CONFIG_SERIAL_8250_PCI and then wonder why no console output
> > > > happened
> > > > on boot. S/he shouldn't have to know that devices were moved
> > > > from
> > > > the
> > > > PCI driver to an LPSS shim driver or that the 8250_lpss driver
> > > > now
> > > > needs to be selected instead of the intuitively correct
> > > > 8250_pci
> > > > driver.
> > > 
> > > That is taken care of since default is set to SERIAL_8250 (you
> > > even
> > > don't need to have PCI driver enabled!).
> > > Doesn't work for you?
> > 
> > The default may be set to SERIAL_8250 but, without the QRK specific
> > entry in 8250_pci.c you won't get console output.
> > 
> > So if you are going to remove the QRK entry from 8250_pci.c and
> > stuff
> > it into 8250_lpss.c then 8250_lpss needs to be selected by
> > CONFIG_SERIAL_8250_PCI.
> 
> Why?!
> 
> Now it should be enough to have SERIAL_8250 set to non-n to have
> 8250_lpss compiled.
> Can you check it?

I'm sure that's true.

My point to you is that - its a highly non-intuitive thing to do on a
reading of the datasheet for this part.

LPSS is, ostensibly at least, for passing processor resources via APCI.

If you look at a QRK datasheet it says "enumerate all this stuff via
PCI" - there's not a single mention of LPSS. Its reasonable, correct
and currently required for QRK to set CONFIG_8250_PCI.

To move away from a valid/standard PCI probe routine into a new special
LPSS/PCI shim (which the hardware doesn't actually mandate) I do think
you should to setup the dependency CONFIG_8250_PCI => CONFIG_8250_LPSS.

---
bod