linux-kernel - [PATCH v2] w1: fix the resume command API

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Message-Id: <20190321105255.19126-1-manio@skyboo.net>
Date:   Thu, 21 Mar 2019 11:52:55 +0100
From:   Mariusz Bialonczyk <manio@...boo.net>
To:     linux-kernel@...r.kernel.org, Evgeniy Polyakov <zbr@...emap.net>,
        Greg Kroah-Hartman <greg@...ah.com>,
        Jean-Francois Dagenais <jeff.dagenais@...il.com>
Cc:     Mariusz Bialonczyk <manio@...boo.net>
Subject: [PATCH v2] w1: fix the resume command API

>From the DS2408 datasheet [1]:
"Resume Command function checks the status of the RC flag and, if it is set,
 directly transfers control to the control functions, similar to a Skip ROM
 command. The only way to set the RC flag is through successfully executing
 the Match ROM, Search ROM, Conditional Search ROM, or Overdrive-Match ROM
 command"

The function currently works perfectly fine in a multidrop bus, but when we
have only a single slave connected, then only a Skip ROM is used and Match
ROM is not called at all. This is leading to problems e.g. with single one
DS2408 connected, as the Resume Command is not working properly and the
device is responding with failing results after the Resume Command.

This commit is fixing this by using a Skip ROM instead in those cases.
The bandwidth / performance advantage is exactly the same.

Refs:
[1] https://datasheets.maximintegrated.com/en/ds/DS2408.pdf

Signed-off-by: Mariusz Bialonczyk <manio@...boo.net>
Reviewed-by: Jean-Francois Dagenais <jeff.dagenais@...il.com>
---
 drivers/w1/w1_io.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/w1/w1_io.c b/drivers/w1/w1_io.c
index 0364d3329c52..3516ce6718d9 100644
--- a/drivers/w1/w1_io.c
+++ b/drivers/w1/w1_io.c
@@ -432,8 +432,7 @@ int w1_reset_resume_command(struct w1_master *dev)
 	if (w1_reset_bus(dev))
 		return -1;
 
-	/* This will make only the last matched slave perform a skip ROM. */
-	w1_write_8(dev, W1_RESUME_CMD);
+	w1_write_8(dev, dev->slave_count > 1 ? W1_RESUME_CMD : W1_SKIP_ROM);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(w1_reset_resume_command);
-- 
2.19.0.rc1