On Fri, Dec 07, 2012 at 04:02:02PM +0900, Alex Courbot wrote: > Hi Guenter, > > On Friday 07 December 2012 10:49:47 Guenter Roeck wrote: > > My own idea for a solution was to keep integer based handles, but replace > > gpio_desc[] with a hash table. But ultimately I don't really care how > > it is done. > > > > Do you have a solution for gpiochip_find_base() in mind, and how to handle > > reservations ? I had thought about using bit maps, but maybe there is > > a better solution. > > My plan so far is to use a sorted linked list of gpio_chips. Each chip > contains its base address and size, so this will make it possible to find > usable areas through a single parse. Current gpiochip_find_base() start from Excellent idea. > ARCH_NR_GPIOS and look backwards in the integer space to find a free range, a > similar behavior can also be done if this is deemed better (GPIO numbers might > become high, but since we want to hide them this should not matter). > You can not completely hide them, I would guess - you'd still want to export them. Anyway, a simpler method would be to keep the list sorted in increasing order and simply search for a large enough gap. If there is none, add the new chip to the end of the list. > The counterpart of the list is that fetching the descriptor corresponding to a > GPIO number is going to be linear instead of constant, but (1) the number of > gpio_chips on the system should never grow very high and (2) this is a good > incentive to use the descriptor-based API instead. :) Existing code could > easily be converted - once a GPIO is acquired, its number should be converted > immediatly to a descriptor and the gpiod_* functions used from them on. We can > probably write a sed or Coccinelle rule to do that through the whole kernel. > Since the current approach loops through all gpio pins, it is still much better than that - the complexity would be O(chips), not O(ngpios). As you said, there won't be many chips, so that should scale for a long time. > gpiochip_reserve() will require some more thinking using this model, but > something like a dummy chip can probably be introduced in the list. It will > need to be statically allocated however since memory allocation cannot be used > there. > Introducing a dummy chip sounds like a good idea, and would have very low overhead. Thanks, Guenter -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
