From: Dave H. <dhy...@gm...> - 2008-12-31 15:22:14
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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/ |