Re: [PATCH v1 06/10] device property: switch to use UUID API

From: "Huang\, Ying" <ying.huang@intel.com>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: linux-efi@vger.kernel.org, David Airlie <airlied@linux.ie>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	dri-devel@lists.freedesktop.org,
	Benjamin Tissoires <benjamin.tissoires@redhat.com>,
	Lv Zheng <lv.zheng@intel.com>,
	Matt Fleming <matt@codeblueprint.co.uk>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-nvdimm@lists.01.org, linux-acpi@vger.kernel.org,
	Huang Ying <ying.huang@intel.com>,
	Zhang Rui <rui.zhang@intel.com>,
	Mathias Nyman <mathias.nyman@intel.com>,
	Jani Nikula <jani.nikula@linux.intel.com>,
	Mark Brown <broonie@kernel.org>,
	Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Theodore Ts'o <tytso@mit.edu>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v1 06/10] device property: switch to use UUID API
Date: Fri, 08 Apr 2016 09:27:12 +0800	[thread overview]
Message-ID: <877fg828lr.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <1460047286.6620.26.camel@linux.intel.com> (Andy Shevchenko's message of "Thu, 7 Apr 2016 19:41:26 +0300")

Andy Shevchenko <andriy.shevchenko@linux.intel.com> writes:

> On Fri, 2016-02-26 at 16:11 +0200, Andy Shevchenko wrote:
>> On Thu, 2016-02-18 at 01:03 +0100, Rafael J. Wysocki wrote:
>> > 
>> > On Wednesday, February 17, 2016 02:17:24 PM Andy Shevchenko wrote:
>> > > 
>> > > Switch to use a generic UUID API instead of custom approach. It
>> > > allows to
>> > > define UUIDs, compare them, and validate.
>> []
>> 
>
> Summon initial author of the UUID library.
>
> Summary: the API of comparison functions is rather strange. What the
> point to not take pointers directly? (Moreover I hope compiler too
> clever not to make a copy of constant arguments there)
>
> I could only imagine the case you are trying to avoid temporary
> variables for constants like NULL_UUID.
>
> Issue with this is the ugliness in the users of that, in particularly
> present in ACPI (drivers/acpi/apei/ghes.c).
>
> I would like to have more clear interface for that. Perhaps we may add
> something like
>
> cmp_p(pointer, non-pointer);
> cmp_pp(pointer, pointer);
>
> to not break existing API for now.
>
> It would be useful for many cases in the kernel.

You can take a look at the drivers/acpi/apei/erst.c for uuid_le_cmp
usage.

#define CPER_CREATOR_PSTORE                                             \
        UUID_LE(0x75a574e3, 0x5052, 0x4b29, 0x8a, 0x8e, 0xbe, 0x2c,     \
                0x64, 0x90, 0xb8, 0x9d)

        if (uuid_le_cmp(rcd->hdr.creator_id, CPER_CREATOR_PSTORE) != 0)
                goto skip;

Looks better?

This is the typical use case in mind when I write the uuid.h.

As for uuid_le_cmp usage in drivers/acpi/apei/ghes.c,

		if (!uuid_le_cmp(*(uuid_le *)gdata->section_type,
				 CPER_SEC_PLATFORM_MEM)) {

The code looks not good mainly because acpi_hest_generic_data is not
defined with uuid_le in mind.

struct acpi_hest_generic_data {
	u8 section_type[16];
	u32 error_severity;
	u16 revision;
	u8 validation_bits;
	u8 flags;
	u32 error_data_length;
	u8 fru_id[16];
	u8 fru_text[20];
};

If section_type was defined as uuid_le instead of u8[16], the
uuid_le_cmp usage would look better.  So I suggest to use uuid_le/be in
data structure definition in new code if possible.

Best Regards,
Huang, Ying

>> > 
>> > > 
>> > > +static const uuid_le ads_uuid =
>> > > +	UUID_LE(0xdbb8e3e6, 0x5886, 0x4ba6,
>> > > +		0x87, 0x95, 0x13, 0x19, 0xf5, 0x2a, 0x96, 0x6b);
>> > >  
>> > >  static bool acpi_enumerate_nondev_subnodes(acpi_handle scope,
>> > >  					   const union
>> > > acpi_object
>> > > *desc,
>> > > @@ -138,7 +136,7 @@ static bool
>> > > acpi_enumerate_nondev_subnodes(acpi_handle scope,
>> > >  		    || links->type != ACPI_TYPE_PACKAGE)
>> > >  			break;
>> > >  
>> > > -		if (memcmp(uuid->buffer.pointer, ads_uuid,
>> > > sizeof(ads_uuid)))
>> > > +		if (uuid_le_cmp(*(uuid_le *)uuid->buffer.pointer,
>> > > ads_uuid))
>> > Maybe it's too late, but I don't quite understand the pointer
>> > manipulations here.
>> > 
>> > I can see why you need a type conversion (although it looks ugly),
>> > but why do you
>> > need to dereference it too?
>> The function takes that kind of type on input. The other variants are
>> not compiled.
>> Perhaps we better change uuid_{lb}e_cmp() first to take normal
>> pointers, though I think the initial idea was to get type checking at
>> compile time.
>> 
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