linux-kernel - Re: [PATCH v1 5/5] pci: keystone: add pcie driver based on designware core driver

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Date:	Tue, 20 May 2014 11:17:25 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	Murali Karicheri <m-karicheri2@...com>,
	"Strashko, Grygorii" <grygorii.strashko@...com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Jingoo Han <jg1.han@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	Mohit Kumar <mohit.kumar@...com>
Subject: Re: [PATCH v1 5/5] pci: keystone: add pcie driver based on designware
 core driver

On Tue, May 20, 2014 at 1:55 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Monday 19 May 2014 17:10:50 Murali Karicheri wrote:

>> On ARM, by default pci_bus_config seems to be set to 0
>> (PCIE_BUS_TUNE_OFF). So the code doesn't get
>> executed for this default. But for PCIE_BUS_SAFE, it doesn't change the
>> mrrs at the EP and is not
>> good for our platform w.r.t mrrs settings. For PCIE_BUS_PERFORMANCE, it
>> seems to increase the payload
>> size as well and Keystone Payload size is limited to 128 bytes. So it is
>> not safe to increase the payload
>> size to 256 based on the log.
>>
>> On other platforms, Why the PCI core try to set the payload size equal
>> to mrrs? Is this explained in any
>> PCI spec?  Looks like this is done for performance? Let me know if you
>> want me to send a patch for
>> review to add the pcie_bus_configure_settings() code to
>> arch/arm/kernel/bios32.c
>>
>> For the Keystone PCI driver, I believe. it is safe to have the quirk so
>> that controller can handle the
>> read requests properly. Let me know if the quirk code above looks good
>> to go.
>
> I don't know enough about these to give you a definite answer, but
> I'd still prefer to handle this in generic code. A quirk seems to
> be the wrong answer here. If this isn't something we can do in generic
> fashion, can you do it in the add_bus() callback perhaps?

I definitely prefer a more generic approach than a quirk.
Unfortunately the MPS management was implemented originally just for
x86, not because there's anything arch-specific about it, but because
the author was only concerned about x86.  Making that more generic has
been an open issue all along.

Bjorn
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