[PATCH v16 11/11] RFC: fw_cfg: do DMA read operation

Marc-André Lureau
2018-02-28T15:07:13Z

Modify fw_cfg_read_blob() to use DMA if the device supports it.
Return errors, because the operation may fail.

So far, only one call in fw_cfg_register_dir_entries() is using
kmalloc'ed buf and is thus clearly eligible to DMA read.

Initially, I didn't implement DMA read to speed up boot time, but as a
first step before introducing DMA write (since read operations were
already presents). Even more, I didn't realize fw-cfg entries were
being read by the kernel during boot by default. But actally fw-cfg
entries are being populated during module probe. I knew DMA improved a
lot bios boot time (the main reason the DMA interface was added
afaik). Let see the time it would take to read the whole ACPI
tables (128kb allocated)

# time cat /sys/firmware/qemu_fw_cfg/by_name/etc/acpi/tables/raw
- with DMA: sys 0m0.003s
- without DMA (-global fw_cfg.dma_enabled=off): sys 0m7.674s

FW_CFG_FILE_DIR (0x19) is the only "file" that is read during kernel
boot to populate sysfs qemu_fw_cfg directory, and it is quite
small (1-2kb). Since it does not expose itself, in order to measure
the time it takes to read such small file, I took a comparable sized
file of 2048 bytes and exposed it (-fw_cfg test,file=file with a
modified read_raw enabling DMA)

# perf stat -r 100 cat /sys/firmware/qemu_fw_cfg/by_name/test/raw >/dev/null
- with DMA:
0.636037 task-clock (msec) # 0.141 CPUs utilized ( +- 1.19% )
- without DMA:
6.430128 task-clock (msec) # 0.622 CPUs utilized ( +- 0.22% )

That's a few msec saved during boot by enabling DMA read (the gain
would be more substantial if other & bigger fw-cfg entries are read by
others from sysfs, unfortunately, it's not clear if we can always
enable DMA there)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
drivers/firmware/qemu_fw_cfg.c | 68 ++++++++++++++++++++++++++++++++++--------
1 file changed, 55 insertions(+), 13 deletions(-)

diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index 14fedbeca724..db1cba4f99bd 100644
--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c
@@ -66,7 +66,6 @@ static void fw_cfg_sel_endianness(u16 key)
iowrite16(key, fw_cfg_reg_ctrl);
}

-#ifdef CONFIG_CRASH_CORE
static inline bool fw_cfg_dma_enabled(void)
{
return (fw_cfg_rev & FW_CFG_VERSION_DMA) && fw_cfg_reg_dma;
@@ -124,14 +123,49 @@ static ssize_t fw_cfg_dma_transfer(void *address, u32 length, u32 control)

return ret;
}
-#endif
+
+/* with acpi & dev locks taken */
+static ssize_t fw_cfg_read_blob_dma(u16 key,
+ void *buf, loff_t pos, size_t count)
+{
+ ssize_t ret;
+
+ if (pos == 0) {
+ ret = fw_cfg_dma_transfer(buf, count, key << 16
+ | FW_CFG_DMA_CTL_SELECT
+ | FW_CFG_DMA_CTL_READ);
+ } else {
+ fw_cfg_sel_endianness(key);
+ ret = fw_cfg_dma_transfer(NULL, pos, FW_CFG_DMA_CTL_SKIP);
+ if (ret < 0)
+ return ret;
+ ret = fw_cfg_dma_transfer(buf, count,
+ FW_CFG_DMA_CTL_READ);
+ }
+
+ return ret;
+}
+
+/* with acpi & dev locks taken */
+static ssize_t fw_cfg_read_blob_io(u16 key,
+ void *buf, loff_t pos, size_t count)
+{
+ fw_cfg_sel_endianness(key);
+ while (pos-- > 0)
+ ioread8(fw_cfg_reg_data);
+ ioread8_rep(fw_cfg_reg_data, buf, count);
+ return count;
+}

/* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
static ssize_t fw_cfg_read_blob(u16 key,
- void *buf, loff_t pos, size_t count)
+ void *buf, loff_t pos, size_t count,
+ ssize_t (*readfn)(u16 key, void *buf,
+ loff_t pos, size_t count))
{
u32 glk = -1U;
acpi_status status;
+ ssize_t ret;

/* If we have ACPI, ensure mutual exclusion against any potential
* device access by the firmware, e.g. via AML methods:
@@ -145,14 +179,19 @@ static ssize_t fw_cfg_read_blob(u16 key,
}

mutex_lock(&fw_cfg_dev_lock);
- fw_cfg_sel_endianness(key);
- while (pos-- > 0)
- ioread8(fw_cfg_reg_data);
- ioread8_rep(fw_cfg_reg_data, buf, count);
+
+ /* fallback to IO if DMA is not available */
+ if (readfn == fw_cfg_read_blob_dma && !fw_cfg_dma_enabled()) {
+ readfn = fw_cfg_read_blob_io;
+ }
+
+ ret = readfn(key, buf, pos, count);
+
mutex_unlock(&fw_cfg_dev_lock);

acpi_release_global_lock(glk);
- return count;
+
+ return ret;
}

#ifdef CONFIG_CRASH_CORE
@@ -286,7 +325,7 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)

/* verify fw_cfg device signature */
if (fw_cfg_read_blob(FW_CFG_SIGNATURE, sig,
- 0, FW_CFG_SIG_SIZE) < 0 ||
+ 0, FW_CFG_SIG_SIZE, fw_cfg_read_blob_io) < 0 ||
memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
fw_cfg_io_cleanup();
return -ENODEV;
@@ -470,7 +509,9 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
if (count > entry->size - pos)
count = entry->size - pos;

- return fw_cfg_read_blob(entry->select, buf, pos, count);
+ /* do not use DMA, virt_to_phys(buf) might not be ok */
+ return fw_cfg_read_blob(entry->select, buf, pos, count,
+ fw_cfg_read_blob_io);
}

static struct bin_attribute fw_cfg_sysfs_attr_raw = {
@@ -636,7 +677,7 @@ static int fw_cfg_register_dir_entries(void)
size_t dir_size;

ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count,
- 0, sizeof(files_count));
+ 0, sizeof(files_count), fw_cfg_read_blob_io);
if (ret < 0)
return ret;

@@ -648,7 +689,7 @@ static int fw_cfg_register_dir_entries(void)
return -ENOMEM;

ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, dir,
- sizeof(files_count), dir_size);
+ sizeof(files_count), dir_size, fw_cfg_read_blob_dma);
if (ret < 0)
goto end;

@@ -699,7 +740,8 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
goto err_probe;

/* get revision number, add matching top-level attribute */
- err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
+ err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev),
+ fw_cfg_read_blob_io);
if (err < 0)
goto err_probe;

--
2.16.1.73.g5832b7e9f2