linux-kernel - [PATCH v3] usb: typec: ucsi: Wait 20ms before reading CCI after a reset

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Message-Id: <20240409-ucsi-reset-delay-v3-1-8440710b012b@chromium.org>
Date: Tue, 09 Apr 2024 01:58:57 +0000
From: Pavan Holla <pholla@...omium.org>
To: Heikki Krogerus <heikki.krogerus@...ux.intel.com>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Abhishek Pandit-Subedi <abhishekpandit@...omium.org>, 
 Pavan Holla <pholla@...omium.org>, Prashant Malani <pmalani@...omium.org>
Subject: [PATCH v3] usb: typec: ucsi: Wait 20ms before reading CCI after a
 reset

The PPM might take time to process a reset. Allow 20ms for the reset to
be processed before reading the CCI.

This should not slow down existing implementations because they would
not set any bits in the CCI after a reset, and would take a 20ms delay
to read the CCI anyway. This change makes the delay explicit, and
reduces a CCI read. Based on the spec, the PPM has 10ms to set busy, so,
20ms seems like a reasonable delay before we read the CCI.

Signed-off-by: Pavan Holla <pholla@...omium.org>
Reviewed-by: Prashant Malani <pmalani@...omium.org>
---
The PPM might take time to process a reset and set the CCI. Give the PPM
20ms to update it's CCI.

Commit message now summarizes discussion in v1.
---
Changes in v3:
- Commit message updated.
- Link to v2: https://lore.kernel.org/r/20240403-ucsi-reset-delay-v2-1-244c175825a4@chromium.org

Changes in v2:
- Commit message updated.
- Link to v1: https://lore.kernel.org/r/20240325-ucsi-reset-delay-v1-1-d9e183e0f1e6@chromium.org
---
 drivers/usb/typec/ucsi/ucsi.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/typec/ucsi/ucsi.c b/drivers/usb/typec/ucsi/ucsi.c
index cf52cb34d285..c0706c40fa64 100644
--- a/drivers/usb/typec/ucsi/ucsi.c
+++ b/drivers/usb/typec/ucsi/ucsi.c
@@ -1280,6 +1280,9 @@ static int ucsi_reset_ppm(struct ucsi *ucsi)
 			goto out;
 		}
 
+		/* Give the PPM time to process a reset before reading CCI */
+		msleep(20);
+
 		ret = ucsi->ops->read(ucsi, UCSI_CCI, &cci, sizeof(cci));
 		if (ret)
 			goto out;
@@ -1293,7 +1296,6 @@ static int ucsi_reset_ppm(struct ucsi *ucsi)
 				goto out;
 		}
 
-		msleep(20);
 	} while (!(cci & UCSI_CCI_RESET_COMPLETE));
 
 out:

---
base-commit: 4cece764965020c22cff7665b18a012006359095
change-id: 20240325-ucsi-reset-delay-bdf6712455fd

Best regards,
-- 
Pavan Holla <pholla@...omium.org>