[PATCH V4 1/5] ACPI / CPPC: Optimize PCC Read Write operations

Prashanth Prakash
2016-02-17T20:21:27Z

From: Ashwin Chaugule <ashwin.chaugule@linaro.org>

Previously the send_pcc_cmd() code checked if the
PCC operation had completed before returning from
the function. This check was performed regardless
of the PCC op type (i.e. Read/Write). Knowing
the type of cmd can be used to optimize the check
and avoid needless waiting. e.g. with Write ops,
the actual Writing is done before calling send_pcc_cmd().
And the subsequent Writes will check if the channel is
free at the entry of send_pcc_cmd() anyway.

However, for Read cmds, we need to wait for the cmd
completion bit to be flipped, since the actual Read
ops follow after returning from the send_pcc_cmd(). So,
only do the looping check at the end for Read ops.

Also, instead of using udelay() calls, use ktime as a
means to check for deadlines. The current deadline
in which the Remote should flip the cmd completion bit
is defined as N * Nominal latency. Where N is arbitrary
and large enough to work on slow emulators and Nominal
latency comes from the ACPI table (PCCT). This helps
in working around the CONFIG_HZ effects on udelay()
and also avoids needing different ACPI tables for Silicon
and Emulation platforms.

Signed-off-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Signed-off-by: Prashanth Prakash <pprakash@codeaurora.org>
---
drivers/acpi/cppc_acpi.c | 101 +++++++++++++++++++++++++++++++++--------------
1 file changed, 72 insertions(+), 29 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 6730f96..026cdd4 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -39,6 +39,7 @@

#include <linux/cpufreq.h>
#include <linux/delay.h>
+#include <linux/ktime.h>

#include <acpi/cppc_acpi.h>
/*
@@ -63,25 +64,53 @@ static struct mbox_chan *pcc_channel;
static void __iomem *pcc_comm_addr;
static u64 comm_base_addr;
static int pcc_subspace_idx = -1;
-static u16 pcc_cmd_delay;
static bool pcc_channel_acquired;
+static ktime_t deadline;

/*
* Arbitrary Retries in case the remote processor is slow to respond
- * to PCC commands.
+ * to PCC commands. Keeping it high enough to cover emulators where
+ * the processors run painfully slow.
*/
#define NUM_RETRIES 500

+static int check_pcc_chan(void)
+{
+ int ret = -EIO;
+ struct acpi_pcct_shared_memory __iomem *generic_comm_base = pcc_comm_addr;
+ ktime_t next_deadline = ktime_add(ktime_get(), deadline);
+
+ /* Retry in case the remote processor was too slow to catch up. */
+ while (!ktime_after(ktime_get(), next_deadline)) {
+ if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
+ ret = 0;
+ break;
+ }
+ /*
+ * Reducing the bus traffic in case this loop takes longer than
+ * a few retries.
+ */
+ udelay(3);
+ }
+
+ return ret;
+}
+
static int send_pcc_cmd(u16 cmd)
{
- int retries, result = -EIO;
- struct acpi_pcct_hw_reduced *pcct_ss = pcc_channel->con_priv;
+ int ret = -EIO;
struct acpi_pcct_shared_memory *generic_comm_base =
(struct acpi_pcct_shared_memory *) pcc_comm_addr;
- u32 cmd_latency = pcct_ss->latency;

- /* Min time OS should wait before sending next command. */
- udelay(pcc_cmd_delay);
+ /*
+ * For CMD_WRITE we know for a fact the caller should have checked
+ * the channel before writing to PCC space
+ */
+ if (cmd == CMD_READ) {
+ ret = check_pcc_chan();
+ if (ret)
+ return ret;
+ }

/* Write to the shared comm region. */
writew(cmd, &generic_comm_base->command);
@@ -90,31 +119,30 @@ static int send_pcc_cmd(u16 cmd)
writew(0, &generic_comm_base->status);

/* Ring doorbell */
- result = mbox_send_message(pcc_channel, &cmd);
- if (result < 0) {
+ ret = mbox_send_message(pcc_channel, &cmd);
+ if (ret < 0) {
pr_err("Err sending PCC mbox message. cmd:%d, ret:%d\n",
- cmd, result);
- return result;
+ cmd, ret);
+ return ret;
}

- /* Wait for a nominal time to let platform process command. */
- udelay(cmd_latency);
-
- /* Retry in case the remote processor was too slow to catch up. */
- for (retries = NUM_RETRIES; retries > 0; retries--) {
- if (readw_relaxed(&generic_comm_base->status) & PCC_CMD_COMPLETE) {
- result = 0;
- break;
- }
- }
+ /*
+ * For READs we need to ensure the cmd completed to ensure
+ * the ensuing read()s can proceed. For WRITEs we dont care
+ * because the actual write()s are done before coming here
+ * and the next READ or WRITE will check if the channel
+ * is busy/free at the entry of this call.
+ */
+ if (cmd == CMD_READ)
+ ret = check_pcc_chan();

- mbox_client_txdone(pcc_channel, result);
- return result;
+ mbox_client_txdone(pcc_channel, ret);
+ return ret;
}

static void cppc_chan_tx_done(struct mbox_client *cl, void *msg, int ret)
{
- if (ret)
+ if (ret < 0)
pr_debug("TX did not complete: CMD sent:%x, ret:%d\n",
*(u16 *)msg, ret);
else
@@ -306,6 +334,7 @@ static int register_pcc_channel(int pcc_subspace_idx)
{
struct acpi_pcct_hw_reduced *cppc_ss;
unsigned int len;
+ u64 usecs_lat;

if (pcc_subspace_idx >= 0) {
pcc_channel = pcc_mbox_request_channel(&cppc_mbox_cl,
@@ -335,7 +364,14 @@ static int register_pcc_channel(int pcc_subspace_idx)
*/
comm_base_addr = cppc_ss->base_address;
len = cppc_ss->length;
- pcc_cmd_delay = cppc_ss->min_turnaround_time;
+
+ /*
+ * cppc_ss->latency is just a Nominal value. In reality
+ * the remote processor could be much slower to reply.
+ * So add an arbitrary amount of wait on top of Nominal.
+ */
+ usecs_lat = NUM_RETRIES * cppc_ss->latency;
+ deadline = ns_to_ktime(usecs_lat * NSEC_PER_USEC);

pcc_comm_addr = acpi_os_ioremap(comm_base_addr, len);
if (!pcc_comm_addr) {
@@ -604,7 +640,7 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
(ref_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) ||
(nom_perf->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM)) {
/* Ring doorbell once to update PCC subspace */
- if (send_pcc_cmd(CMD_READ)) {
+ if (send_pcc_cmd(CMD_READ) < 0) {
ret = -EIO;
goto out_err;
}
@@ -662,7 +698,7 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
if ((delivered_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) ||
(reference_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM)) {
/* Ring doorbell once to update PCC subspace */
- if (send_pcc_cmd(CMD_READ)) {
+ if (send_pcc_cmd(CMD_READ) < 0) {
ret = -EIO;
goto out_err;
}
@@ -713,6 +749,13 @@ int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)

spin_lock(&pcc_lock);

+ /* If this is PCC reg, check if channel is free before writing */
+ if (desired_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) {
+ ret = check_pcc_chan();
+ if (ret)
+ goto busy_channel;
+ }
+
/*
* Skip writing MIN/MAX until Linux knows how to come up with
* useful values.
@@ -722,10 +765,10 @@ int cppc_set_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls)
/* Is this a PCC reg ?*/
if (desired_reg->cpc_entry.reg.space_id == ACPI_ADR_SPACE_PLATFORM_COMM) {
/* Ring doorbell so Remote can get our perf request. */
- if (send_pcc_cmd(CMD_WRITE))
+ if (send_pcc_cmd(CMD_WRITE) < 0)
ret = -EIO;
}
-
+busy_channel:
spin_unlock(&pcc_lock);

return ret;