Re: [RFC 2.6.28 1/1] gpiolib: add set/get batch v3

Hi Jaya,

...snip...

There was a question about why have both bitmask AND width when width
could be worked out from bitmask. The reason for having both bitmask and
length is for performance. Let's compare the following two scenarios:

/* flash to black */
for (i=0; i < 800*600; i++)
       gpio_set_batch(DB0, 0x00000000, 0x0000FFFF)

Internally, the above implementation has to work out the length of bits
that are being set in bitmask in order to figure out which register
boundaries are being crossed. That means looping through the bitmask
and counting bits. That would need to be repeated for every call of
gpio_set_batch in the above loop and would be a relatively high expense.

/* flash to black */
for (i=0; i < 800*600; i++)
       gpio_set_batch(DB0, 0x00000000, 0x0000FFFF, 16)

In the 2nd case, the implementation is able to skip all the bit counting.
This usage method is also the intuitive way for drivers that are trying
to push data across a gpio bus since they explicitly know the bus length.

...snip...

I haven't been following too closely, but what about

        gpio_set_batch(DB0, 0x00000000, 0x00FF00FF, 16)

That also has 16 bits set, but in different positions.

Shouldn't you just not bother with the length and optimize particular
bitmasks (i.e. 0xFF, 0xFFFF, 0xFFFFFF, 0xFFFFFFFF)?

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.DaveHylands.com/