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

[Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>]

On Fri, 2016-04-08 at 09:27 +0800, Huang, Ying wrote:
> Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/CA@public.gmane.org> 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?

I don't quite understand the issues with 

if (uuid_le_cmp(&rcd->hdr.creator_id, &CPER_CREATOR_PSTORE) != 0)

or, like I mentioned previously, we may introduce _cmp_p() and use like

if (uuid_le_cmp_p(&rcd->hdr.creator_id, CPER_CREATOR_PSTORE) != 0)

if it looks better (again, I don't know if compiler is going to copy the last argument).

> 
> 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)) {

Ditto

if (!uuid_le_cmp_p((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.

This is understandable for such structures, but we might get a UUID from
a buffer which is pointer to u8. It's not possible to convert to uuid_*
since it's too generic stuff and might require to introduce
ACPI_TYPE_UUID with standardization and all necessary work. Apparently
not the shortest way.

> 
> 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.
> > > 

-- 
Andy Shevchenko <andriy.shevchenko-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Intel Finland Oy