arm: mvebu: a38x: serdes: Add comments for hws_pex_config() code

Add comments to understand what this magic code is doing.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>

diff --git a/arch/arm/mach-mvebu/serdes/a38x/ctrl_pex.c b/arch/arm/mach-mvebu/serdes/a38x/ctrl_pex.c
index 717bcfb29cc1eb238595eaa3c7b16e3dc182def7..0eb31d589ca3612d81488b104d242d97acd4f612 100644 (file)
--- a/arch/arm/mach-mvebu/serdes/a38x/ctrl_pex.c
+++ b/arch/arm/mach-mvebu/serdes/a38x/ctrl_pex.c
@@ -42,6 +42,7 @@ int hws_pex_config(const struct serdes_map *serdes_map, u8 count)
                        continue;
                }
 
+               /* Set Device/Port Type to RootComplex */
                pex_idx = serdes_type - PEX0;
                tmp = reg_read(PEX_CAPABILITIES_REG(pex_idx));
                tmp &= ~(0xf << 20);
@@ -122,12 +123,18 @@ int hws_pex_config(const struct serdes_map *serdes_map, u8 count)
                }
 
                next_busno++;
+
+               /*
+                * Read maximum link speed. It must be 0x2 (5.0 GT/s) as this
+                * value was set in serdes_power_up_ctrl() function.
+                */
                temp_pex_reg = reg_read((PEX_CFG_DIRECT_ACCESS
                                         (pex_idx, PEX_LINK_CAPABILITY_REG)));
                temp_pex_reg &= 0xf;
                if (temp_pex_reg != 0x2)
                        continue;
 
+               /* Read negotiated link speed */
                temp_reg = (reg_read(PEX_CFG_DIRECT_ACCESS(
                                             pex_idx,
                                             PEX_LINK_CTRL_STAT_REG)) &
@@ -155,6 +162,7 @@ int hws_pex_config(const struct serdes_map *serdes_map, u8 count)
                        continue;
                }
 
+               /* Find start of the PCI Express Capability registers */
                while ((pex_config_read(pex_idx, first_busno, 0, 0, addr)
                        & 0xff) != 0x10) {
                        addr = (pex_config_read(pex_idx, first_busno, 0,
@@ -173,11 +181,15 @@ int hws_pex_config(const struct serdes_map *serdes_map, u8 count)
                tmp = reg_read(PEX_LINK_CTRL_STATUS2_REG(pex_idx));
                DEBUG_RD_REG(PEX_LINK_CTRL_STATUS2_REG(pex_idx), tmp);
                tmp &= ~(BIT(0) | BIT(1));
-               tmp |= BIT(1);
+               tmp |= BIT(1);  /* Force Target Link Speed to 5.0 GT/s */
                tmp |= BIT(6);  /* Select Deemphasize (-3.5d_b) */
                reg_write(PEX_LINK_CTRL_STATUS2_REG(pex_idx), tmp);
                DEBUG_WR_REG(PEX_LINK_CTRL_STATUS2_REG(pex_idx), tmp);
 
+               /*
+                * Enable Auto Speed change. When set, link will issue link
+                * speed change to max possible link speed.
+                */
                tmp = reg_read(PEX_CTRL_REG(pex_idx));
                DEBUG_RD_REG(PEX_CTRL_REG(pex_idx), tmp);
                tmp |= BIT(10);