[PATCH 1/2] staging/vc04_services: Derive g_cache_line_size

[Phil Elwell <phil@raspberrypi.org>]

The ARM coprocessor registers include dcache line size, but there is no
function to expose this value. Rather than create a new one, use the
read_cpuid_id function to derive the correct value, which is 32 for
BCM2835 and 64 for BCM2836/7.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
---
 .../interface/vchiq_arm/vchiq_2835_arm.c           | 24 +++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
index e767209..3537f60 100644
--- a/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
+++ b/drivers/staging/vc04_services/interface/vchiq_arm/vchiq_2835_arm.c
@@ -42,6 +42,7 @@
 #include <linux/uaccess.h>
 #include <linux/mm.h>
 #include <linux/of.h>
+#include <asm/cputype.h>
 #include <soc/bcm2835/raspberrypi-firmware.h>
 
 #define TOTAL_SLOTS (VCHIQ_SLOT_ZERO_SLOTS + 2 * 32)
@@ -81,13 +82,15 @@ static void __iomem *g_regs;
  * VPU firmware, which determines the required alignment of the
  * offsets/sizes in pagelists.
  *
- * Modern VPU firmware looks for a DT "cache-line-size" property in
- * the VCHIQ node and will overwrite it with the actual L2 cache size,
+ * Previous VPU firmware looked for a DT "cache-line-size" property in
+ * the VCHIQ node and would overwrite it with the actual L2 cache size,
  * which the kernel must then respect.  That property was rejected
- * upstream, so we have to use the VPU firmware's compatibility value
- * of 32.
+ * upstream, so we now rely on both sides to "do the right thing" independently
+ * of the other. To improve backwards compatibility, this new behaviour is
+ * signalled to the firmware by the use of a corrected "reg" property on the
+ * relevant Device Tree node.
  */
-static unsigned int g_cache_line_size = 32;
+static unsigned int g_cache_line_size;
 static unsigned int g_fragments_size;
 static char *g_fragments_base;
 static char *g_free_fragments;
@@ -127,6 +130,17 @@ int vchiq_platform_init(struct platform_device *pdev, VCHIQ_STATE_T *state)
 	if (err < 0)
 		return err;
 
+	/*
+	 * The tempting L1_CACHE_BYTES macro doesn't work in the case of
+	 * a kernel built with bcm2835_defconfig running on a BCM2836/7
+	 * processor, hence the need for a runtime check. The dcache line size
+	 * is encoded in one of the coprocessor registers, but there is no
+	 * convenient way to access it short of embedded assembler, hence
+	 * the use of read_cpuid_id(). The following test evaluates to true
+	 * on a BCM2835 showing that it is ARMv6-ish, whereas
+	 * cpu_architecture() will indicate that it is an ARMv7.
+	 */
+	g_cache_line_size = ((read_cpuid_id() & 0x7f000) == 0x7b000) ? 32 : 64;
 	g_fragments_size = 2 * g_cache_line_size;
 
 	/* Allocate space for the channels in coherent memory */
-- 
2.7.4