[PATCH] SPI: busnum == 0 needs to work

We need to be able to have a "SPI bus 0" matching chip numbering; but
that number was wrongly used to flag dynamic allocation of a bus number.

This patch resolves that issue; now negative numbers trigger dynamic alloc.

It also updates the how-to-write-a-controller-driver overview to mention
this stuff.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 1168ef015887..7a3f733051e9 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -395,7 +395,7 @@ EXPORT_SYMBOL_GPL(spi_alloc_master);
 int __init_or_module
 spi_register_master(struct spi_master *master)
 {
-        static atomic_t         dyn_bus_id = ATOMIC_INIT(0);
+        static atomic_t         dyn_bus_id = ATOMIC_INIT((1<<16) - 1);
         struct device           *dev = master->cdev.dev;
         int                     status = -ENODEV;
         int                     dynamic = 0;
@@ -404,7 +404,7 @@ spi_register_master(struct spi_master *master)
                 return -ENODEV;
 
         /* convention:  dynamically assigned bus IDs count down from the max */
-        if (master->bus_num == 0) {
+        if (master->bus_num < 0) {
                 master->bus_num = atomic_dec_return(&dyn_bus_id);
                 dynamic = 1;
         }