Re: [PATCH 03/19] ArmPlatformPkg: PL111 and HDLCD: add const qualifier

[Evan Lloyd <Evan.Lloyd@arm.com>]

Hi Ard.
Response inline below

> -----Original Message-----
> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
> Sent: 12 October 2017 20:47
> To: Evan Lloyd <Evan.Lloyd@arm.com>
> Cc: edk2-devel@lists.01.org; "ard.biesheuvel@linaro.org"@arm.com;
> "leif.lindholm@linaro.org"@arm.com;
> "Matteo.Carlini@arm.com"@arm.com; "nd@arm.com"@arm.com
> Subject: Re: [PATCH 03/19] ArmPlatformPkg: PL111 and HDLCD: add const
> qualifier
>
> On 26 September 2017 at 21:15,  <evan.lloyd@arm.com> wrote:
> > From: Girish Pathak <girish.pathak@arm.com>
> >
> > This change adds some STATIC and CONST qualifiers (mainly to arguments
> > of  functions) in PL111 and HdLcd modules.
> >
> > It doesn't add or modify any functionality.
> >
> > Contributed-under: TianoCore Contribution Agreement 1.1
> > Signed-off-by: Girish Pathak <girish.pathak@arm.com>
> > Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
> > ---
> >
> ArmPlatformPkg/ArmVExpressPkg/Library/HdLcdArmVExpressLib/HdLcdAr
> mVExpress.c       | 34 ++++++++++----------
> >
> ArmPlatformPkg/ArmVExpressPkg/Library/PL111LcdArmVExpressLib/PL11
> 1LcdArmVExpress.c | 34 ++++++++++----------
> >  ArmPlatformPkg/Drivers/LcdGraphicsOutputDxe/HdLcd.c
> |  4 +--
> >  ArmPlatformPkg/Drivers/LcdGraphicsOutputDxe/PL111Lcd.c
> |  4 +--
> >  4 files changed, 38 insertions(+), 38 deletions(-)
> >
> > diff --git
> >
> a/ArmPlatformPkg/ArmVExpressPkg/Library/HdLcdArmVExpressLib/HdLcd
> ArmVE
> > xpress.c
> >
> b/ArmPlatformPkg/ArmVExpressPkg/Library/HdLcdArmVExpressLib/HdLcd
> ArmVE
> > xpress.c index
> >
> cfe3259d3c737de240350e8c3eab867b80c40948..b9859a56988f7e5be0ad
> baa49048
> > a683fe586bfe 100644
> > ---
> >
> a/ArmPlatformPkg/ArmVExpressPkg/Library/HdLcdArmVExpressLib/HdLcd
> ArmVE
> > xpress.c
> > +++
> b/ArmPlatformPkg/ArmVExpressPkg/Library/HdLcdArmVExpressLib/HdLcd
> A
> > +++ rmVExpress.c
> > @@ -46,7 +46,7 @@ typedef struct {
> >
> >  /** The display modes supported by the platform.
> >  **/
> > -LCD_RESOLUTION mResolutions[] = {
> > +STATIC CONST LCD_RESOLUTION mResolutions[] = {
> >    { // Mode 0 : VGA : 640 x 480 x 24 bpp
> >      VGA, VGA_H_RES_PIXELS, VGA_V_RES_PIXELS,
> LCD_BITS_PER_PIXEL_24,
> >      VGA_OSC_FREQUENCY,
> > @@ -144,8 +144,8 @@ LcdPlatformInitializeDisplay (  **/  EFI_STATUS
> > LcdPlatformGetVram (
> > -  OUT EFI_PHYSICAL_ADDRESS*  VramBaseAddress,
> > -  OUT UINTN*                 VramSize
> > +  OUT EFI_PHYSICAL_ADDRESS * CONST  VramBaseAddress,
> > +  OUT UINTN * CONST                 VramSize
>
> What is the point of this CONST (and all the other occurrences in this patch)
>
> In all cases [AFAICT] the CONST applies to the argument itself, not to the
> object it points to, which means the variable is CONST in the scope of the
> function, but can still be dereferenced to assign the OUT value.
>
> This means your change is technically correct, but it is extremely
> unidiomatic for EDK2, so an explanation why this driver needs this would be
> highly appreciated.
>
[[Evan Lloyd]] The style is explicitly sanctioned by the Edk2 CCS § 5.6.2.4.2
" Constant pointer to variable: UINTN * CONST ConstPointer;
    ConstPointer is a constant pointer to a variable UINTN."

The real benefit is that it clearly identifies the pointer as not changed in the function.
In this specific instance that also makes it obvious that the OUT parameters are not array bases, just pointers to individual values.

On a broader note - why would you ever not have a const where something is not modified?

As another view, the "unidiomatic for EDK2" argument implies you have a very high opinion of the existing ArmPlatformPkg code quality.
The  "we have always done it that way" argument does not encourage quality improvements.

Regards,
Evan
> >    )
...
> > --
> > Guid("CE165669-3EF3-493F-B85D-6190EE5B9759")
> >
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