gpio: userspace ABI for reading/writing GPIO lines
This adds a userspace ABI for reading and writing GPIO lines. The mechanism returns an anonymous file handle to a request to read/write n offsets from a gpiochip. This file handle in turn accepts two ioctl()s: one that reads and one that writes values to the selected lines. - Handles can be requested as input/output, active low, open drain, open source, however when you issue a request for n lines with GPIO_GET_LINEHANDLE_IOCTL, they must all have the same flags, i.e. all inputs or all outputs, all open drain etc. If a granular control of the flags for each line is desired, they need to be requested individually, not in a batch. - The GPIOHANDLE_GET_LINE_VALUES_IOCTL read ioctl() can be issued also to output lines to verify that the hardware is in the expected state. - It reads and writes up to GPIOHANDLES_MAX lines at once, utilizing the .set_multiple() call in the driver if possible, making the call efficient if several lines can be written with a single register update. The limitation of GPIOHANDLES_MAX to 64 lines is done under the assumption that we may expect hardware that can issue a transaction updating 64 bits at an instant but unlikely anything larger than that. ChangeLog v2->v3: - Use gpiod_get_value_cansleep() so we support also slowpath GPIO drivers. - Fix up the UAPI docs kerneldoc. - Allocate the anonymous fd last, so that the release function don't get called until that point of something fails. After this point, skip the errorpath. ChangeLog v1->v2: - Handle ioctl_compat() properly based on a similar patch to the other ioctl() handling code. - Use _IOWR() as we pass pointers both in and out of the ioctl() - Use kmalloc() and kfree() for the linehandled, do not try to be fancy with devm_* it doesn't work the way I thought. - Fix const-correctness on the linehandle name field. Acked-by:Michael Welling <mwelling@ieee.org> Signed-off-by: