iwlwifi: Add comments about MAC_ACCESS_REQ

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/drivers/net/wireless/iwlwifi/iwl-io.h b/drivers/net/wireless/iwlwifi/iwl-io.h
index 0a078b082833..d0a358c9d96b 100644
--- a/drivers/net/wireless/iwlwifi/iwl-io.h
+++ b/drivers/net/wireless/iwlwifi/iwl-io.h
@@ -200,6 +200,26 @@ static inline int _iwl_grab_nic_access(struct iwl_priv *priv)
 
         /* this bit wakes up the NIC */
         _iwl_set_bit(priv, CSR_GP_CNTRL, CSR_GP_CNTRL_REG_FLAG_MAC_ACCESS_REQ);
+
+        /*
+         * These bits say the device is running, and should keep running for
+         * at least a short while (at least as long as MAC_ACCESS_REQ stays 1),
+         * but they do not indicate that embedded SRAM is restored yet;
+         * 3945 and 4965 have volatile SRAM, and must save/restore contents
+         * to/from host DRAM when sleeping/waking for power-saving.
+         * Each direction takes approximately 1/4 millisecond; with this
+         * overhead, it's a good idea to grab and hold MAC_ACCESS_REQUEST if a
+         * series of register accesses are expected (e.g. reading Event Log),
+         * to keep device from sleeping.
+         *
+         * CSR_UCODE_DRV_GP1 register bit MAC_SLEEP == 0 indicates that
+         * SRAM is okay/restored.  We don't check that here because this call
+         * is just for hardware register access; but GP1 MAC_SLEEP check is a
+         * good idea before accessing 3945/4965 SRAM (e.g. reading Event Log).
+         *
+         * 5000 series and later (including 1000 series) have non-volatile SRAM,
+         * and do not save/restore SRAM when power cycling.
+         */
         ret = _iwl_poll_bit(priv, CSR_GP_CNTRL,
                            CSR_GP_CNTRL_REG_VAL_MAC_ACCESS_EN,
                            (CSR_GP_CNTRL_REG_FLAG_MAC_CLOCK_READY |