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.lur...@redhat.com>
---
 drivers/firmware/qemu_fw_cfg.c | 80 ++++++++++++++++++++++++++++++++++--------
 1 file changed, 66 insertions(+), 14 deletions(-)

diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
index 69939e2529f2..ba9b907a4399 100644
--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c
@@ -124,12 +124,46 @@ static ssize_t fw_cfg_dma_transfer(void *address, u32 
length, u32 control)
        return ret;
 }
 
+/* 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 {
+               iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
+               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 void fw_cfg_read_blob_io(u16 key,
+                               void *buf, loff_t pos, size_t count)
+{
+       iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
+       while (pos-- > 0)
+               ioread8(fw_cfg_reg_data);
+       ioread8_rep(fw_cfg_reg_data, buf, count);
+}
+
 /* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
-static inline void fw_cfg_read_blob(u16 key,
-                                   void *buf, loff_t pos, size_t count)
+static ssize_t fw_cfg_read_blob(u16 key,
+                               void *buf, loff_t pos, size_t count,
+                               bool dma)
 {
        u32 glk = -1U;
        acpi_status status;
+       ssize_t ret = count;
 
        /* If we have ACPI, ensure mutual exclusion against any potential
         * device access by the firmware, e.g. via AML methods:
@@ -139,17 +173,21 @@ static inline void fw_cfg_read_blob(u16 key,
                /* Should never get here */
                WARN(1, "fw_cfg_read_blob: Failed to lock ACPI!\n");
                memset(buf, 0, count);
-               return;
+               return -EINVAL;
        }
 
        mutex_lock(&fw_cfg_dev_lock);
-       iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
-       while (pos-- > 0)
-               ioread8(fw_cfg_reg_data);
-       ioread8_rep(fw_cfg_reg_data, buf, count);
+       if (dma && fw_cfg_dma_enabled()) {
+               ret = fw_cfg_read_blob_dma(key, buf, pos, count);
+       } else {
+               fw_cfg_read_blob_io(key, buf, pos, count);
+       }
+
        mutex_unlock(&fw_cfg_dev_lock);
 
        acpi_release_global_lock(glk);
+
+       return ret;
 }
 
 #ifdef CONFIG_CRASH_CORE
@@ -282,8 +320,9 @@ static int fw_cfg_do_platform_probe(struct platform_device 
*pdev)
 #endif
 
        /* verify fw_cfg device signature */
-       fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE);
-       if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
+       if (fw_cfg_read_blob(FW_CFG_SIGNATURE, sig,
+                               0, FW_CFG_SIG_SIZE, false) < 0 ||
+               memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
                fw_cfg_io_cleanup();
                return -ENODEV;
        }
@@ -466,8 +505,8 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, 
struct kobject *kobj,
        if (count > entry->size - pos)
                count = entry->size - pos;
 
-       fw_cfg_read_blob(entry->select, buf, pos, count);
-       return count;
+       /* do not use DMA, virt_to_phys(buf) might not be ok */
+       return fw_cfg_read_blob(entry->select, buf, pos, count, false);
 }
 
 static struct bin_attribute fw_cfg_sysfs_attr_raw = {
@@ -632,7 +671,12 @@ static int fw_cfg_register_dir_entries(void)
        struct fw_cfg_file *dir;
        size_t dir_size;
 
-       fw_cfg_read_blob(FW_CFG_FILE_DIR, &files.count, 0, sizeof(files.count));
+       ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, &files.count,
+                       0, sizeof(files.count), false);
+       if (ret < 0) {
+               return ret;
+       }
+       
        count = be32_to_cpu(files.count);
        dir_size = count * sizeof(struct fw_cfg_file);
 
@@ -640,7 +684,11 @@ static int fw_cfg_register_dir_entries(void)
        if (!dir)
                return -ENOMEM;
 
-       fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(files.count), dir_size);
+       ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, dir,
+                       sizeof(files.count), dir_size, false);
+       if (ret < 0) {
+               goto end;
+       }
 
        for (i = 0; i < count; i++) {
                ret = fw_cfg_register_file(&dir[i]);
@@ -648,6 +696,7 @@ static int fw_cfg_register_dir_entries(void)
                        break;
        }
 
+end:
        kfree(dir);
        return ret;
 }
@@ -688,7 +737,10 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
                goto err_probe;
 
        /* get revision number, add matching top-level attribute */
-       fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
+       err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev), false);
+       if (err < 0) {
+               goto err_probe;
+       }
        fw_cfg_rev = le32_to_cpu(rev);
        err = sysfs_create_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
        if (err)
-- 
2.16.1.73.g5832b7e9f2

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