cxl: Fix driver use count

cxl keeps a driver use count, which is used with the hash memory model
on p8 to know when to upgrade local TLBIs to global and to trigger
callbacks to manage the MMU for PSL8.

If a process opens a context and closes without attaching or fails the
attachment, the driver use count is never decremented. As a
consequence, TLB invalidations remain global, even if there are no
active cxl contexts.

We should increment the driver use count when the process is attaching
to the cxl adapter, and not on open. It's not needed before the
adapter starts using the context and the use count is decremented on
the detach path, so it makes more sense.

It affects only the user api. The kernel api is already doing The
Right Thing.
Signed-off-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org # v4.2+
Fixes: 7bb5d91a ("cxl: Rework context lifetimes")
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/misc/cxl/api.c

@@ -336,6 +336,10 @@ int cxl_start_context(struct cxl_context *ctx, u64 wed,
 			mmput(ctx->mm);
 	}
 
+	/*
+	 * Increment driver use count. Enables global TLBIs for hash
+	 * and callbacks to handle the segment table
+	 */
 	cxl_ctx_get();
 
 	if ((rc = cxl_ops->attach_process(ctx, kernel, wed, 0))) {

drivers/misc/cxl/file.c

@@ -95,7 +95,6 @@ static int __afu_open(struct inode *inode, struct file *file, bool master)
 
 	pr_devel("afu_open pe: %i\n", ctx->pe);
 	file->private_data = ctx;
-	cxl_ctx_get();
 
 	/* indicate success */
 	rc = 0;
@@ -225,6 +224,12 @@ static long afu_ioctl_start_work(struct cxl_context *ctx,
 	if (ctx->mm)
 		mmput(ctx->mm);
 
+	/*
+	 * Increment driver use count. Enables global TLBIs for hash
+	 * and callbacks to handle the segment table
+	 */
+	cxl_ctx_get();
+
 	trace_cxl_attach(ctx, work.work_element_descriptor, work.num_interrupts, amr);
 
 	if ((rc = cxl_ops->attach_process(ctx, false, work.work_element_descriptor,
@@ -233,6 +238,7 @@ static long afu_ioctl_start_work(struct cxl_context *ctx,
 		cxl_adapter_context_put(ctx->afu->adapter);
 		put_pid(ctx->pid);
 		ctx->pid = NULL;
+		cxl_ctx_put();
 		cxl_context_mm_count_put(ctx);
 		goto out;
 	}