Anastasia Klimchuk submitted this change.

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buspirate_spi: add support for hiz output with pullups=off

When working with low-voltage chips, the internal 10k pull-ups
of the Bus Pirate might be too high. In such cases, it's necessary
to create an external pull-up using lower-value resistors. For this,
you can use the 'hiz' parameter.  This way, the Bus Pirate will
operate as an open drain with 'pullups=off'

So, why not just use the 'pullups=on' option for this scenario? I'm
trying to prevent a very typical human error in the training sessions
I conduct.

For example, in the previous session, a user might have left the VPU
(vextern) connected to 5V, and now they need to access a 1.8V chip.
If 'pullups=on' is used, the following will happen:
5V (VPU) --> CD4060 --> 2K (internal Bus Pirate) --> MOSI target chip.
By having the option to set a 'hiz=on', this can be avoided. Since the
pull-ups will remain deactivated while the Bus Pirate will function in
an open-drain mode

Return init error for invalid values of pullups, hiz, psus. Previously,
invalid values of the params pullups, hiz, psus were handled as "off"
(i.e. disabled). This created a potential human error when users were
running flashrom with e.g. pullups=1 and expected pullups to be on,
while the value 1 was handled as invalid and off.

Change-Id: I6d9909b23782ef3fcb1428fd127dae0d6b211855
Signed-off-by: David Reguera Garcia <dreg@rootkit.es>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/79299
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
---
M buspirate_spi.c
M doc/classic_cli_manpage.rst
2 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/buspirate_spi.c b/buspirate_spi.c
index 72c28b0..3a1e414 100644
--- a/buspirate_spi.c
+++ b/buspirate_spi.c
@@ -325,6 +325,7 @@
 	int spispeed = 0x7;
 	int serialspeed_index = -1;
 	int ret = 0;
+	bool hiz = false;
 	bool pullup = false;
 	bool psu = false;
 	bool aux = true;
@@ -370,23 +371,49 @@
 
 	tmp = extract_programmer_param_str(cfg, "pullups");
 	if (tmp) {
-		if (strcasecmp("on", tmp) == 0)
+		if (strcasecmp("on", tmp) == 0) {
 			pullup = true;
-		else if (strcasecmp("off", tmp) == 0)
+		} else if (strcasecmp("off", tmp) == 0) {
 			; // ignore
-		else
-			msg_perr("Invalid pullups state, not using them.\n");
+		} else {
+			msg_perr("Invalid pullups state. Use on/off.\n");
+			free(tmp);
+			return 1;
+		}
+	}
+	free(tmp);
+
+	tmp = extract_programmer_param_str(cfg, "hiz");
+	if (tmp) {
+		if (strcasecmp("on", tmp) == 0) {
+			hiz = true;
+		} else if (strcasecmp("off", tmp) == 0) {
+			if (pullup) {
+				msg_perr("Invalid combination: pullups=on & hiz=off at same time is not possible.\n");
+				free(tmp);
+				return 1;
+			} else {
+				; // ignore
+			}
+		} else {
+			msg_perr("Invalid hiz state. Use on/off.\n");
+			free(tmp);
+			return 1;
+		}
 	}
 	free(tmp);
 
 	tmp = extract_programmer_param_str(cfg, "psus");
 	if (tmp) {
-		if (strcasecmp("on", tmp) == 0)
+		if (strcasecmp("on", tmp) == 0) {
 			psu = true;
-		else if (strcasecmp("off", tmp) == 0)
+		} else if (strcasecmp("off", tmp) == 0) {
 			; // ignore
-		else
-			msg_perr("Invalid psus state, not enabling.\n");
+		} else {
+			msg_perr("Invalid psus state. Use on/off.\n");
+			free(tmp);
+			return 1;
+		}
 	}
 	free(tmp);
 
@@ -692,9 +719,9 @@
 
 	/* Set SPI config: output type, idle, clock edge, sample */
 	bp_commbuf[0] = 0x80 | 0xa;
-	if (pullup) {
+	if (pullup || hiz) {
 		bp_commbuf[0] &= ~(1 << 3);
-		msg_pdbg("Pull-ups enabled, so using HiZ pin output! (Open-Drain mode)\n");
+		msg_pdbg("Pull-ups or HiZ enabled, so using HiZ pin output! (Open-Drain mode)\n");
 	}
 	ret = buspirate_sendrecv(bp_commbuf, 1, 1);
 	if (ret)
diff --git a/doc/classic_cli_manpage.rst b/doc/classic_cli_manpage.rst
index 5d6b7ef..0cd45ca 100644
--- a/doc/classic_cli_manpage.rst
+++ b/doc/classic_cli_manpage.rst
@@ -772,6 +772,14 @@
 More information about the Bus Pirate pull-up resistors and their purpose is available
 `in a guide by dangerousprototypes <http://dangerousprototypes.com/docs/Practical_guide_to_Bus_Pirate_pull-up_resistors>`_.
 
+When working with low-voltage chips, the internal 10k pull-ups of the Bus Pirate might be too high. In such cases, it's necessary to create an external pull-up using lower-value resistors.
+
+For this, you can use the ``hiz`` parameter. This way, the Bus Pirate will operate as an open drain. Syntax is::
+
+        flashrom -p buspirate_spi:hiz=state
+
+where ``state`` can be ``on`` or ``off``.
+
 The state of the Bus Pirate power supply pins is controllable through an optional ``psus`` parameter. Syntax is::
 
         flashrom -p buspirate_spi:psus=state

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