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ACPI / CPPC: Optimize PCC Read Write operations 

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>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Ashwin Chaugule 2016-02-17 13:20:59 -07:00 committed by Rafael J. Wysocki
parent 2db8f9a1d8
commit ad62e1e677
1 changed files with 72 additions and 29 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

101
drivers/acpi/cppc_acpi.c
View File

@ -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);
/*
* 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();
/* 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;
}
}
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;