Re: [PATCH] gpio: gpio-rcar: Support S2RAM

[Simon Horman <horms@xxxxxxxxxxxx>]

On Thu, Dec 07, 2017 at 10:26:53AM +0100, Geert Uytterhoeven wrote:
> On Thu, Dec 7, 2017 at 10:21 AM, Geert Uytterhoeven
> <geert@xxxxxxxxxxxxxx> wrote:
> >>> >  struct gpio_rcar_priv {
> >>> >         void __iomem *base;
> >>> >         spinlock_t lock;
> >>> > @@ -41,6 +51,7 @@ struct gpio_rcar_priv {
> >>> >         unsigned int irq_parent;
> >>> >         bool has_both_edge_trigger;
> >>> >         bool needs_clk;
> >>> > +       struct gpio_rcar_bank_info bank_info[32];
> >>>
> >>> That's 32 x 7 = 224 bytes in total.
> >>>
> >>> What about just using 7 u32s instead, one for each register to save?
> >>> That way you only need 7 x 4 = 28 bytes, and you can probably optimize
> >>> the code to just save/restore the whole register at once.
> >>
> >> So the suggestion is to use a u32 instead of struct gpio_rcar_bank_info,
> >> and for each field of struct gpio_rcar_bank_info use a bit in the u32?
> >>
> >> If so, probably one could go a step further and use a u8 as there are
> >> currently only 7 fields, thus using 32 x 1 = 32 bytes rather than
> >> 32 x 4 = 128 bytes.
> >
> > I think you misunderstood.
> > The patch has one gpio_rcar_bank_info for each GPIO.
> > Each bank has 7 bits (bools), one for each register.
> > Indexing is done through bank_info[<gpio>].<reg>.
> > Saving/restoring bits requires converting from hardware register layout to
> > stored layout ("transposing a 32 x 7 matrix to a 7 x 32 matrix").
> >
> > I proposed 7 u32s, one for each register, storing the similar bits for all
> > 32 GPIOs.
> > So indexing is reversed, becoming regs[<reg>] & BIT(<gpio>), which is
> > similar to how the data is stored in hardware registers.
> > Storing all bits related to a single register in a single u32 may allow to
> > save/restore all bits of the register in a single operation.
> 
> More clarification: it's the difference between "int array[7][32]" and
> "int array[32][7]".  Both store the same amount of data.
> But if the hardware uses the former organization, you want to
> save/restore using the same organization, else it requires an expensive
> transformation.

Thanks, you are correct that I misunderstood.
I understand now.

Kaneko-san, could you take a look at switching this around and posting an RFT?
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