[PATCH V4 3/5] mailbox: pcc: optimized pcc_send_data

Prashanth Prakash
2016-02-17T20:21:26Z

pcc_send_data can be invoked during the execution of performance
critical code as in cppc_cpufreq driver. With acpi_* APIs, the
doorbell register accessed in pcc_send_data if present in system
memory will be searched (in cached virt to phys addr mapping),
mapped, read/written and then unmapped. These operations take
significant amount of time.

This patch maps the performance critical doorbell register
during init and then reads/writes to it directly using the
mapped virtual address. This patch + similar changes to CPPC
acpi driver reduce the time per freq. transition from around
200us to about 20us for cppc cpufreq driver

Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
Acked-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
---
drivers/mailbox/pcc.c | 111 ++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 104 insertions(+), 7 deletions(-)

diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
index 8f779a1..0ddf638 100644
--- a/drivers/mailbox/pcc.c
+++ b/drivers/mailbox/pcc.c
@@ -63,6 +63,7 @@
#include <linux/platform_device.h>
#include <linux/mailbox_controller.h>
#include <linux/mailbox_client.h>
+#include <linux/io-64-nonatomic-lo-hi.h>

#include "mailbox.h"

@@ -70,6 +71,9 @@

static struct mbox_chan *pcc_mbox_channels;

+/* Array of cached virtual address for doorbell registers */
+static void __iomem **pcc_doorbell_vaddr;
+
static struct mbox_controller pcc_mbox_ctrl = {};
/**
* get_pcc_channel - Given a PCC subspace idx, get
@@ -160,6 +164,66 @@ void pcc_mbox_free_channel(struct mbox_chan *chan)
}
EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);

+/*
+ * PCC can be used with perf critical drivers such as CPPC
+ * So it makes sense to locally cache the virtual address and
+ * use it to read/write to PCC registers such as doorbell register
+ *
+ * The below read_register and write_registers are used to read and
+ * write from perf critical registers such as PCC doorbell register
+ */
+static int read_register(void __iomem *vaddr, u64 *val, unsigned int bit_width)
+{
+ int ret_val = 0;
+
+ switch (bit_width) {
+ case 8:
+ *val = readb(vaddr);
+ break;
+ case 16:
+ *val = readw(vaddr);
+ break;
+ case 32:
+ *val = readl(vaddr);
+ break;
+ case 64:
+ *val = readq(vaddr);
+ break;
+ default:
+ pr_debug("Error: Cannot read register of %u bit width",
+ bit_width);
+ ret_val = -EFAULT;
+ break;
+ }
+ return ret_val;
+}
+
+static int write_register(void __iomem *vaddr, u64 val, unsigned int bit_width)
+{
+ int ret_val = 0;
+
+ switch (bit_width) {
+ case 8:
+ writeb(val, vaddr);
+ break;
+ case 16:
+ writew(val, vaddr);
+ break;
+ case 32:
+ writel(val, vaddr);
+ break;
+ case 64:
+ writeq(val, vaddr);
+ break;
+ default:
+ pr_debug("Error: Cannot write register of %u bit width",
+ bit_width);
+ ret_val = -EFAULT;
+ break;
+ }
+ return ret_val;
+}
+
/**
* pcc_send_data - Called from Mailbox Controller code. Used
* here only to ring the channel doorbell. The PCC client
@@ -175,21 +239,39 @@ EXPORT_SYMBOL_GPL(pcc_mbox_free_channel);
static int pcc_send_data(struct mbox_chan *chan, void *data)
{
struct acpi_pcct_hw_reduced *pcct_ss = chan->con_priv;
- struct acpi_generic_address doorbell;
+ struct acpi_generic_address *doorbell;
u64 doorbell_preserve;
u64 doorbell_val;
u64 doorbell_write;
+ u32 id = chan - pcc_mbox_channels;
+ int ret = 0;
+
+ if (id >= pcc_mbox_ctrl.num_chans) {
+ pr_debug("pcc_send_data: Invalid mbox_chan passed\n");
+ return -ENOENT;
+ }

- doorbell = pcct_ss->doorbell_register;
+ doorbell = &pcct_ss->doorbell_register;
doorbell_preserve = pcct_ss->preserve_mask;
doorbell_write = pcct_ss->write_mask;

/* Sync notification from OS to Platform. */
- acpi_read(&doorbell_val, &doorbell);
- acpi_write((doorbell_val & doorbell_preserve) | doorbell_write,
- &doorbell);
-
- return 0;
+ if (pcc_doorbell_vaddr[id]) {
+ ret = read_register(pcc_doorbell_vaddr[id], &doorbell_val,
+ doorbell->bit_width);
+ if (ret)
+ return ret;
+ ret = write_register(pcc_doorbell_vaddr[id],
+ (doorbell_val & doorbell_preserve) | doorbell_write,
+ doorbell->bit_width);
+ } else {
+ ret = acpi_read(&doorbell_val, doorbell);
+ if (ret)
+ return ret;
+ ret = acpi_write((doorbell_val & doorbell_preserve) | doorbell_write,
+ doorbell);
+ }
+ return ret;
}

static const struct mbox_chan_ops pcc_chan_ops = {
@@ -265,14 +347,29 @@ static int __init acpi_pcc_probe(void)
return -ENOMEM;
}

+ pcc_doorbell_vaddr = kcalloc(count, sizeof(void *), GFP_KERNEL);
+ if (!pcc_doorbell_vaddr) {
+ kfree(pcc_mbox_channels);
+ return -ENOMEM;
+ }
+
/* Point to the first PCC subspace entry */
pcct_entry = (struct acpi_subtable_header *) (
(unsigned long) pcct_tbl + sizeof(struct acpi_table_pcct));

for (i = 0; i < count; i++) {
+ struct acpi_generic_address *db_reg;
+ struct acpi_pcct_hw_reduced *pcct_ss;
pcc_mbox_channels[i].con_priv = pcct_entry;
pcct_entry = (struct acpi_subtable_header *)
((unsigned long) pcct_entry + pcct_entry->length);
+
+ /* If doorbell is in system memory cache the virt address */
+ pcct_ss = (struct acpi_pcct_hw_reduced *)pcct_entry;
+ db_reg = &pcct_ss->doorbell_register;
+ if (db_reg->space_id == ACPI_ADR_SPACE_SYSTEM_MEMORY)
+ pcc_doorbell_vaddr[i] = acpi_os_ioremap(db_reg->address,
+ db_reg->bit_width/8);
}

pcc_mbox_ctrl.num_chans = count;