mmc_spi: Fix mmc-over-spi regression

Patch 49dce689ad4ef0fd1f970ef762168e4bd46f69a3 changed the sysfs data
structures for SPI in a way which broke the MMC-over-SPI host driver.

This patch fixes that regression by changing the scheme used to keep
from knowingly trying to use a shared bus segment, and updates the
adjacent comments slightly to better explain the issue.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>

1 files changed, 33 insertions, 19 deletions

diff --git a/drivers/mmc/host/mmc_spi.c b/drivers/mmc/host/mmc_spi.c
index 12c2d807c145..a6469218f194 100644
--- a/drivers/mmc/host/mmc_spi.c
+++ b/drivers/mmc/host/mmc_spi.c
@@ -1165,6 +1165,23 @@ mmc_spi_detect_irq(int irq, void *mmc)
         return IRQ_HANDLED;
 }
 
+struct count_children {
+        unsigned        n;
+        struct bus_type *bus;
+};
+
+static int maybe_count_child(struct device *dev, void *c)
+{
+        struct count_children *ccp = c;
+
+        if (dev->bus == ccp->bus) {
+                if (ccp->n)
+                        return -EBUSY;
+                ccp->n++;
+        }
+        return 0;
+}
+
 static int mmc_spi_probe(struct spi_device *spi)
 {
         void                    *ones;
@@ -1188,33 +1205,30 @@ static int mmc_spi_probe(struct spi_device *spi)
                 return status;
         }
 
-        /* We can use the bus safely iff nobody else will interfere with
-         * us.  That is, either we have the experimental exclusive access
-         * primitives ... or else there's nobody to share it with.
+        /* We can use the bus safely iff nobody else will interfere with us.
+         * Most commands consist of one SPI message to issue a command, then
+         * several more to collect its response, then possibly more for data
+         * transfer.  Clocking access to other devices during that period will
+         * corrupt the command execution.
+         *
+         * Until we have software primitives which guarantee non-interference,
+         * we'll aim for a hardware-level guarantee.
+         *
+         * REVISIT we can't guarantee another device won't be added later...
          */
         if (spi->master->num_chipselect > 1) {
-                struct device   *parent = spi->dev.parent;
+                struct count_children cc;
 
-                /* If there are multiple devices on this bus, we
-                 * can't proceed.
-                 */
-                spin_lock(&parent->klist_children.k_lock);
-                if (parent->klist_children.k_list.next
-                                != parent->klist_children.k_list.prev)
-                        status = -EMLINK;
-                else
-                        status = 0;
-                spin_unlock(&parent->klist_children.k_lock);
+                cc.n = 0;
+                cc.bus = spi->dev.bus;
+                status = device_for_each_child(spi->dev.parent, &cc,
+                                maybe_count_child);
                 if (status < 0) {
                         dev_err(&spi->dev, "can't share SPI bus\n");
                         return status;
                 }
 
-                /* REVISIT we can't guarantee another device won't
-                 * be added later.  It's uncommon though ... for now,
-                 * work as if this is safe.
-                 */
-                dev_warn(&spi->dev, "ASSUMING unshared SPI bus!\n");
+                dev_warn(&spi->dev, "ASSUMING SPI bus stays unshared!\n");
         }
 
         /* We need a supply of ones to transmit.  This is the only time