[PATCH v4 1/3] thunderbolt: Adjust how NVM reading works

[Mario Limonciello]

Some TBT3 devices have a hard time reliably responding to bit banging
requests correctly when connected to AMD USB4 hosts running Linux.

These problems are not reported in any other CM supported on AMD platforms,
and comparing the Windows and Pre-OS implementations the Linux CM is the
only one that utilizes bit banging to access the DROM.
Other CM implementations access the DROM directly from the NVM instead of
bit banging.

Adjust the flow to use this method to fetch the NVM when the downstream
device has an LC that can be used to fetch the UUID later. The bit banging
method will only be used if this has failed or no LC is present.

Signed-off-by: Mario Limonciello <mario.limonciello@xxxxxxx>
---
v3->v4:
    * Don't run code for USB4 devices
v2->v3:
    * Split out refactor
v1->v2:
    * Update commit message to indicate which CMs are tested
    * Adjust flow to only fetch DROM from NVM on TBT3 and bit bang on TBT1/2
---
 drivers/thunderbolt/eeprom.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/thunderbolt/eeprom.c b/drivers/thunderbolt/eeprom.c
index c90d22f56d4e..8c9e553e2fca 100644
--- a/drivers/thunderbolt/eeprom.c
+++ b/drivers/thunderbolt/eeprom.c
@@ -640,6 +640,11 @@ int tb_drom_read(struct tb_switch *sw)
 		return 0;
 	}
 
+	/* We can use LC to get UUID later */
+	if (sw->cap_lc && !tb_switch_is_usb4(sw) &&
+	    tb_drom_copy_nvm(sw, &size) == 0)
+		goto parse;
+
 	res = tb_drom_read_n(sw, 14, (u8 *) &size, 2);
 	if (res)
 		return res;
-- 
2.34.1