[PATCH v3 1/7] Drivers: hv: vmbus: Add utility function for querying ring size

[Saurabh Sengar <ssengar@linux.microsoft.com>]

Add a function to query for the preferred ring buffer size of VMBus
device. This will allow the drivers (eg. UIO) to allocate the most
optimized ring buffer size for devices.

Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Reviewed-by: Long Li <longli@microsoft.com>
---
[V2]
- Added more details in commit message.
- Added comments for preferred ring sizes and there values.
- Added reviewed-by from Long Li.

 drivers/hv/channel_mgmt.c | 15 ++++++++++++---
 drivers/hv/hyperv_vmbus.h |  5 +++++
 include/linux/hyperv.h    |  2 ++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 2f4d09ce027a..3c6011a48dab 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -120,7 +120,9 @@ const struct vmbus_device vmbus_devs[] = {
 	},
 
 	/* File copy */
-	{ .dev_type = HV_FCOPY,
+	/* fcopy always uses 16KB ring buffer size and is working well for last many years */
+	{ .pref_ring_size = 0x4000,
+	  .dev_type = HV_FCOPY,
 	  HV_FCOPY_GUID,
 	  .perf_device = false,
 	  .allowed_in_isolated = false,
@@ -140,12 +142,19 @@ const struct vmbus_device vmbus_devs[] = {
 	  .allowed_in_isolated = false,
 	},
 
-	/* Unknown GUID */
-	{ .dev_type = HV_UNKNOWN,
+	/*
+	 * Unknown GUID
+	 * 64 KB ring buffer + 4 KB header should be sufficient size for any Hyper-V device apart
+	 * from HV_NIC and HV_SCSI. This case avoid the fallback for unknown devices to allocate
+	 * much bigger (2 MB) of ring size.
+	 */
+	{ .pref_ring_size = 0x11000,
+	  .dev_type = HV_UNKNOWN,
 	  .perf_device = false,
 	  .allowed_in_isolated = false,
 	},
 };
+EXPORT_SYMBOL_GPL(vmbus_devs);
 
 static const struct {
 	guid_t guid;
diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index f6b1e710f805..76ac5185a01a 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -417,6 +417,11 @@ static inline bool hv_is_perf_channel(struct vmbus_channel *channel)
 	return vmbus_devs[channel->device_id].perf_device;
 }
 
+static inline size_t hv_dev_ring_size(struct vmbus_channel *channel)
+{
+	return vmbus_devs[channel->device_id].pref_ring_size;
+}
+
 static inline bool hv_is_allocated_cpu(unsigned int cpu)
 {
 	struct vmbus_channel *channel, *sc;
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 6ef0557b4bff..7de9f90d3f95 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -820,6 +820,8 @@ struct vmbus_requestor {
 #define VMBUS_RQST_RESET (U64_MAX - 3)
 
 struct vmbus_device {
+	/* preferred ring buffer size in KB, 0 means no preferred size for this device */
+	size_t pref_ring_size;
 	u16  dev_type;
 	guid_t guid;
 	bool perf_device;
-- 
2.34.1