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SubjectRe: [PATCH v2 04/31] arm64: MMU definitions
Hi Arnd,

On Wed, Aug 15, 2012 at 02:30:01PM +0100, Arnd Bergmann wrote:
> On Tuesday 14 August 2012, Catalin Marinas wrote:
> > +/*
> > + * TCR flags.
> > + */
> > +#define TCR_TxSZ(x) (((64 - (x)) << 16) | ((64 - (x)) << 0))
> > +#define TCR_IRGN_NC ((0 << 8) | (0 << 24))
> > +#define TCR_IRGN_WBWA ((1 << 8) | (1 << 24))
> > +#define TCR_IRGN_WT ((2 << 8) | (2 << 24))
> > +#define TCR_IRGN_WBnWA ((3 << 8) | (3 << 24))
> > +#define TCR_IRGN_MASK ((3 << 8) | (3 << 24))
> > +#define TCR_ORGN_NC ((0 << 10) | (0 << 26))
> > +#define TCR_ORGN_WBWA ((1 << 10) | (1 << 26))
> > +#define TCR_ORGN_WT ((2 << 10) | (2 << 26))
> > +#define TCR_ORGN_WBnWA ((3 << 10) | (3 << 26))
> > +#define TCR_ORGN_MASK ((3 << 10) | (3 << 26))
> > +#define TCR_SHARED ((3 << 12) | (3 << 28))
> > +#define TCR_TG0_64K (1 << 14)
> > +#define TCR_TG1_64K (1 << 30)
> > +#define TCR_IPS_40BIT (2 << 32)
> > +#define TCR_ASID16 (1 << 36)
> > +
>
> As a matter of coding style, I would much prefer tables like this to be
> written as
>
> #define TCR_IRGN_MASK 0x0000000003000300
> #define TCR_IRGN_WBnWA 0x0000000003000300
> #define TCR_IRGN_WT 0x0000000002000200
> #define TCR_IRGN_WBWA 0x0000000001000100
> #define TCR_IRGN_NC 0x0000000000000000
>
> #define TCR_ORGN_MASK 0x000000000c000c00
> #define TCR_ORGN_WBnWA 0x000000000c000c00
> #define TCR_ORGN_WT 0x0000000008000800
> #define TCR_ORGN_WBWA 0x0000000004000400
> #define TCR_ORGN_NC 0x0000000000000000
>
> The advantage of this is that you can visually compare the bitmasks
> to a hex dump, and if you are suffering from endian-confused documentation
> authors, there is no ambiguity about which end of the word is bit zero.

That depends on the case, in some places it's more readable like this.
In the above case, I find it easier to compare against the documentation
which, for example, has groups of 2 bits at position 8 and 24 or 10 and
26 (for TTBR0 and TTBR1). The meaning of a group of 2 bits is described
separately as 0b00 (NC), 0b01(WBWA) etc. Same goes for the shareability
bits (12 and 28).

So I think at least for code writing it's less error-prone to write the
explicit bit position than a magic long hex.

--
Catalin


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