linux-kernel - Re: [RFC PATCH v4 3/5] PCI: Check platform specific ECAM quirks

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Date:	Wed, 29 Jun 2016 09:34:01 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Tomasz Nowicki <tn@...ihalf.com>, Duc Dang <dhdang@....com>
Cc:	Bjorn Helgaas <helgaas@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Rafael Wysocki <rafael@...nel.org>,
	Hanjun Guo <hanjun.guo@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Sinan Kaya <okaya@...eaurora.org>,
	Jayachandran C <jchandra@...adcom.com>,
	Robert Richter <robert.richter@...iumnetworks.com>,
	Marcin Wojtas <mw@...ihalf.com>,
	Liviu Dudau <Liviu.Dudau@....com>,
	David Daney <ddaney@...iumnetworks.com>,
	Yijing Wang <wangyijing@...wei.com>,
	Mark Salter <msalter@...hat.com>, linux-pci@...r.kernel.org,
	linux-arm <linux-arm-kernel@...ts.infradead.org>,
	linux-acpi@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linaro-acpi@...ts.linaro.org, Jon Masters <jcm@...hat.com>,
	Andrea Gallo <andrea.gallo@...aro.org>, jeremy.linton@....com,
	Dongdong Liu <liudongdong3@...wei.com>,
	Gabriele Paoloni <gabriele.paoloni@...wei.com>,
	Jeffrey Hugo <jhugo@...eaurora.org>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [RFC PATCH v4 3/5] PCI: Check platform specific ECAM quirks

Hi Tomasz,

On 06/29/2016 06:48 AM, Tomasz Nowicki wrote:
> On 28.06.2016 18:12, Duc Dang wrote:
>> On Tue, Jun 28, 2016 at 6:04 AM, Christopher Covington
>> <cov@...eaurora.org> wrote:
>>> Hi Tomasz,

>>> Ard's comments on v3 included:
>>>
>>> "... exact OEM table/rev id matches ..."
>>> "... substring match ... out of the question ..."

Digging through the archives I see Jon Master commented earlier to "be
careful with substring match".

>> I think having OEM Table ID as "PLAT " and then "PLAT2 " (the the 
>> next version of the SoC) is common. So yes, matching full string is
>> better as we can use "PLAT2 " in MCFG table and not worry about the
>> "PLAT" sub-string match causes the quirk to be applied
>> unintentionally.

> Note that platforms already shipped where OEM string has no padding will

I'm confused by this statement. OEMID is defined as 6 bytes long and OEM
Table ID as 8 bytes long in the ACPI specification. As far as I can
tell, if your string isn't exactly that long, padding up to that length
is required.

> have change the firmware or add 0 padding to our quirk array IDs.

The fixed 6 or 8 character string compare, as used v2 of this patchset,
will be compatible with existing firmware as best I can tell. Adding
padding to the quirk array IDs is exactly what I'm suggesting, although
all the strings I've seen are space padded rather than null padded.

Matches:
{"APM   ", "XGENE   ", 1}
{"CAVIUM", "THUNDERX", 1}
{"HISI  ", "HISI-D02", 1}
{"HISI  ", "HISI-D03", 1}
{"QCOM  ", "QDF2432 ", 1}

Given the above tuples, won't accidentally match:
(guessing at possible future ids)
{"APM   ", "XGENEi  ", 1}
{"CAVIUM", "THUNDERX", i} i != 1
{"CAVIUM", "THUNDERi", 1}
{"CAVIUM", "THUNDRXi", 1}
{"HISI  ", "HISI-D0i", 1} i != 2 && i != 3
{"QCOM  ", "QDF24ij ", 1} i != 3 && j != 2

References for APM, HiSilicon IDs:
https://lists.linaro.org/pipermail/linaro-acpi/2016-June/007108.html
https://lists.linaro.org/pipermail/linaro-acpi/2016-June/007043.html

Thanks,
Cov

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