iio: pressure: ms5611: ms5611_prom_is_valid false negative bug [PATCH]

[Alexander Zangerl <az@breathe-safe.com>]

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the ms5611 driver falsely rejects lots of MS5607-02BA03-50 chips
with "PROM integrity check failed" because it doesn't accept a prom crc
value of zero as legitimate.

according to the datasheet for this chip (and the manufacturer's
application note about the prom crc), none of the possible
values for the crc are excluded - but the current code
in ms5611_prom_is_valid() ends with

return crc_orig != 0x0000 && crc == crc_orig

datasheet: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=MS5607-02BA03&DocType=Data+Sheet&DocLang=English
application note: https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=AN520_C-code_example_for_MS56xx&DocType=SS&DocLang=EN

i've discussed this whith the original author of the driver (tomasz
duszynski) and he indicated that at that time (2015) he was dealing with
some faulty chip samples which returned blank data under some
circumstances and/or followed example code which indicated crc zero
being bad (i can't find any traces of any such online, however).

as far as i can tell this exception should not be applied anymore; we've
got a few hundred custom boards here with this chip where large numbers
of the prom have a legitimate crc value 0, and do work fine, but which the
current driver code wrongly rejects.

(i can provide some example prom dumps if required.)

the attached tiny patch is against 4.19. but that
section of the code is unchanged up to and including 6.6. 


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From 653b5cf063e07d126e67386b152e4e76d4f8c1dc Mon Sep 17 00:00:00 2001
From: Alexander Zangerl <az@breathe-safe.com>
Date: Mon, 18 Sep 2023 09:44:00 +1000
Subject: [PATCH] ms5611: crc zero is valid and should not be rejected

---
 drivers/iio/pressure/ms5611_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/iio/pressure/ms5611_core.c b/drivers/iio/pressure/ms5611_core.c
index 5c7a734ede54..9980c6f3335e 100644
--- a/drivers/iio/pressure/ms5611_core.c
+++ b/drivers/iio/pressure/ms5611_core.c
@@ -79,7 +79,7 @@ static bool ms5611_prom_is_valid(u16 *prom, size_t len)
 
 	crc = (crc >> 12) & 0x000F;
 
-	return crc_orig != 0x0000 && crc == crc_orig;
+	return crc == crc_orig;
 }
 
 static int ms5611_read_prom(struct iio_dev *indio_dev)
-- 
2.30.2


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Best Regards,
Alexander Zangerl
IT Engineer

-- 
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