[PATCH v4 0/5] i3c: introduce and use generic parity helper

[PATCH v4 0/5] i3c: introduce and use generic parity helper

[Wolfram Sang]

Changes since v3:

* updated commit message of patch 4 to state that a bug gets fixed.

All acks from bitmap.h and HWMON maintainers are already included.

Old coverletter follows:

I am currently working on upstreaming another I3C controller driver. As
many others, it needs to ensure odd parity for a dynamically assigned
address. The BSP version of the driver implemented a custom parity
algorithm. Wondering why we don't have a generic helper for this in the
kernel, I found that many I3C controller drivers all implement their
version of handling parity.

So, I sent out an RFC[1] moving the efficient implementation of the
SPD5118 driver to a generic location. The series was well received, but
the path for upstream was not clear. Because I need the implementation
for my I3C controller driver and I3C is a prominent user of this new
function, I got the idea of converting the existing I3C drivers and
resend the series, suggesting this all goes upstream via I3C.

A build-tested branch is here:

git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux.git renesas/i3c/get_parity

Looking forward to comments...

[1] https://lore.kernel.org/all/20241214085833.8695-1-wsa+renesas@sang-engineering.com/



Wolfram Sang (5):
  bitops: add generic parity calculation for u8
  hwmon: (spd5118) Use generic parity calculation
  i3c: dw: use parity8 helper instead of open coding it
  i3c: mipi-i3c-hci: use parity8 helper instead of open coding it
  i3c: cdns: use parity8 helper instead of open coding it

 drivers/hwmon/spd5118.c                  |  8 +-----
 drivers/i3c/master/dw-i3c-master.c       | 14 +++--------
 drivers/i3c/master/i3c-master-cdns.c     |  3 +--
 drivers/i3c/master/mipi-i3c-hci/dat_v1.c | 11 +--------
 include/linux/bitops.h                   | 31 ++++++++++++++++++++++++
 5 files changed, 37 insertions(+), 30 deletions(-)

-- 
2.45.2


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[PATCH v4 1/5] bitops: add generic parity calculation for u8

[Wolfram Sang]

There are multiple open coded implementations for getting the parity of
a byte in the kernel, even using different approaches. Take the pretty
efficient version from SPD5118 driver and make it generally available by
putting it into the bitops header. As long as there is just one parity
calculation helper, the creation of a distinct 'parity.h' header was
discarded. Also, the usage of hweight8() for architectures having a
popcnt instruction is postponed until a use case within hot paths is
desired. The motivation for this patch is the frequent use of odd parity
in the I3C specification and to simplify drivers there.

Changes compared to the original SPD5118 version are the addition of
kernel documentation, switching the return type from bool to int, and
renaming the argument of the function.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Yury Norov <yury.norov@gmail.com>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Tested-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 include/linux/bitops.h | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index ba35bbf07798..c1cb53cf2f0f 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -229,6 +229,37 @@ static inline int get_count_order_long(unsigned long l)
 	return (int)fls_long(--l);
 }
 
+/**
+ * parity8 - get the parity of an u8 value
+ * @value: the value to be examined
+ *
+ * Determine the parity of the u8 argument.
+ *
+ * Returns:
+ * 0 for even parity, 1 for odd parity
+ *
+ * Note: This function informs you about the current parity. Example to bail
+ * out when parity is odd:
+ *
+ *	if (parity8(val) == 1)
+ *		return -EBADMSG;
+ *
+ * If you need to calculate a parity bit, you need to draw the conclusion from
+ * this result yourself. Example to enforce odd parity, parity bit is bit 7:
+ *
+ *	if (parity8(val) == 0)
+ *		val ^= BIT(7);
+ */
+static inline int parity8(u8 val)
+{
+	/*
+	 * One explanation of this algorithm:
+	 * https://funloop.org/codex/problem/parity/README.html
+	 */
+	val ^= val >> 4;
+	return (0x6996 >> (val & 0xf)) & 1;
+}
+
 /**
  * __ffs64 - find first set bit in a 64 bit word
  * @word: The 64 bit word
-- 
2.45.2


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[PATCH v4 2/5] hwmon: (spd5118) Use generic parity calculation

[Wolfram Sang]

Make use of the new generic helper for calculating the parity.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 drivers/hwmon/spd5118.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/hwmon/spd5118.c b/drivers/hwmon/spd5118.c
index 6cee48a3e5c3..358152868d96 100644
--- a/drivers/hwmon/spd5118.c
+++ b/drivers/hwmon/spd5118.c
@@ -291,12 +291,6 @@ static umode_t spd5118_is_visible(const void *_data, enum hwmon_sensor_types typ
 	}
 }
 
-static inline bool spd5118_parity8(u8 w)
-{
-	w ^= w >> 4;
-	return (0x6996 >> (w & 0xf)) & 1;
-}
-
 /*
  * Bank and vendor id are 8-bit fields with seven data bits and odd parity.
  * Vendor IDs 0 and 0x7f are invalid.
@@ -304,7 +298,7 @@ static inline bool spd5118_parity8(u8 w)
  */
 static bool spd5118_vendor_valid(u8 bank, u8 id)
 {
-	if (!spd5118_parity8(bank) || !spd5118_parity8(id))
+	if (parity8(bank) == 0 || parity8(id) == 0)
 		return false;
 
 	id &= 0x7f;
-- 
2.45.2


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[PATCH v4 3/5] i3c: dw: use parity8 helper instead of open coding it

[Wolfram Sang]

The kernel has now a generic helper for getting parity with easier to
understand semantics. Make use of it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
---
 drivers/i3c/master/dw-i3c-master.c | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/drivers/i3c/master/dw-i3c-master.c b/drivers/i3c/master/dw-i3c-master.c
index d4b80eb8cecd..0d4d44458c11 100644
--- a/drivers/i3c/master/dw-i3c-master.c
+++ b/drivers/i3c/master/dw-i3c-master.c
@@ -251,14 +251,6 @@ struct dw_i3c_i2c_dev_data {
 	struct i3c_generic_ibi_pool *ibi_pool;
 };
 
-static u8 even_parity(u8 p)
-{
-	p ^= p >> 4;
-	p &= 0xf;
-
-	return (0x9669 >> p) & 1;
-}
-
 static bool dw_i3c_master_supports_ccc_cmd(struct i3c_master_controller *m,
 					   const struct i3c_ccc_cmd *cmd)
 {
@@ -848,7 +840,7 @@ static int dw_i3c_master_daa(struct i3c_master_controller *m)
 	struct dw_i3c_xfer *xfer;
 	struct dw_i3c_cmd *cmd;
 	u32 olddevs, newdevs;
-	u8 p, last_addr = 0;
+	u8 last_addr = 0;
 	int ret, pos;
 
 	ret = pm_runtime_resume_and_get(master->dev);
@@ -873,9 +865,9 @@ static int dw_i3c_master_daa(struct i3c_master_controller *m)
 		}
 
 		master->devs[pos].addr = ret;
-		p = even_parity(ret);
 		last_addr = ret;
-		ret |= (p << 7);
+
+		ret |= parity8(ret) ? 0 : BIT(7);
 
 		writel(DEV_ADDR_TABLE_DYNAMIC_ADDR(ret),
 		       master->regs +
-- 
2.45.2


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[PATCH v4 4/5] i3c: mipi-i3c-hci: use parity8 helper instead of open coding it

[Wolfram Sang]

The kernel has now a generic helper for getting parity with easier to
understand semantics. Make use of it. Here, it also fixes a bug because
the correct algorithm is using XOR ('^=') instead of ADD ('+=').

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
---

Change since v3:

* updated commit message to mention the bugfix

I intentionally did not add a Fixes tag because this fix depends on
patch 1. The proper fix for backporting would change this to XOR, I'd
think.

 drivers/i3c/master/mipi-i3c-hci/dat_v1.c | 11 +----------
 1 file changed, 1 insertion(+), 10 deletions(-)

diff --git a/drivers/i3c/master/mipi-i3c-hci/dat_v1.c b/drivers/i3c/master/mipi-i3c-hci/dat_v1.c
index 47b9b4d4ed3f..85c4916972e4 100644
--- a/drivers/i3c/master/mipi-i3c-hci/dat_v1.c
+++ b/drivers/i3c/master/mipi-i3c-hci/dat_v1.c
@@ -40,15 +40,6 @@
 #define dat_w0_write(i, v)	writel(v, hci->DAT_regs + (i) * 8)
 #define dat_w1_write(i, v)	writel(v, hci->DAT_regs + (i) * 8 + 4)
 
-static inline bool dynaddr_parity(unsigned int addr)
-{
-	addr |= 1 << 7;
-	addr += addr >> 4;
-	addr += addr >> 2;
-	addr += addr >> 1;
-	return (addr & 1);
-}
-
 static int hci_dat_v1_init(struct i3c_hci *hci)
 {
 	unsigned int dat_idx;
@@ -123,7 +114,7 @@ static void hci_dat_v1_set_dynamic_addr(struct i3c_hci *hci,
 	dat_w0 = dat_w0_read(dat_idx);
 	dat_w0 &= ~(DAT_0_DYNAMIC_ADDRESS | DAT_0_DYNADDR_PARITY);
 	dat_w0 |= FIELD_PREP(DAT_0_DYNAMIC_ADDRESS, address) |
-		  (dynaddr_parity(address) ? DAT_0_DYNADDR_PARITY : 0);
+		  (parity8(address) ? 0 : DAT_0_DYNADDR_PARITY);
 	dat_w0_write(dat_idx, dat_w0);
 }
 
-- 
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Re: [PATCH v4 4/5] i3c: mipi-i3c-hci: use parity8 helper instead of open coding it

[Jarkko Nikula]

On 1/7/25 11:02 AM, Wolfram Sang wrote:
> The kernel has now a generic helper for getting parity with easier to
> understand semantics. Make use of it. Here, it also fixes a bug because
> the correct algorithm is using XOR ('^=') instead of ADD ('+=').
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
> Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
> ---
> 
> Change since v3:
> 
> * updated commit message to mention the bugfix
> 
> I intentionally did not add a Fixes tag because this fix depends on
> patch 1. The proper fix for backporting would change this to XOR, I'd
> think.
> 
Stable rules allow also cherry picking additional patches. To me picking 
patch 1 and 4 sounds better than an intermediate fix since bug has been 
here from the beginning. IMHO not so urgent than a regression.

Looks like we have been lucky. First dynamic address is 0x9 and previous 
algorithm gets the same calculated dat_w0 value for at least for the 
addresses 0x9 and 0xa.

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Re: [PATCH v4 4/5] i3c: mipi-i3c-hci: use parity8 helper instead of open coding it

[Wolfram Sang]

[-- Attachment #1.1: Type: text/plain, Size: 740 bytes --]


> Stable rules allow also cherry picking additional patches. To me picking
> patch 1 and 4 sounds better than an intermediate fix since bug has been here
> from the beginning. IMHO not so urgent than a regression.

Even then, doesn't that mean that the series needs to be applied
upstream first before it can go to stable? How to describe the
dependency commit id otherwise? Either Alexandre adds this when
applying, or some interested party ;) sends a backport request to
stable. Or am I missing something?

> Looks like we have been lucky. First dynamic address is 0x9 and previous
> algorithm gets the same calculated dat_w0 value for at least for the
> addresses 0x9 and 0xa.

Makes sense. Still not too many I3C devices out there...


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Re: [PATCH v4 4/5] i3c: mipi-i3c-hci: use parity8 helper instead of open coding it

[Jarkko Nikula]

On 1/8/25 2:56 PM, Wolfram Sang wrote:
> 
>> Stable rules allow also cherry picking additional patches. To me picking
>> patch 1 and 4 sounds better than an intermediate fix since bug has been here
>> from the beginning. IMHO not so urgent than a regression.
> 
> Even then, doesn't that mean that the series needs to be applied
> upstream first before it can go to stable? How to describe the
> dependency commit id otherwise? Either Alexandre adds this when
> applying, or some interested party ;) sends a backport request to
> stable. Or am I missing something?
> 
Yes, the latter case was in my mind that whoever runs this driver on 
earlier stable kernel and has enough devices on the bus where previous 
code will calculate wrong can request the backport or at least be vocal 
about it :-)

I'd say backporting back to 9ad9a52cce28 ("i3c/master: introduce the 
mipi-i3c-hci driver") is also needless because driver needs other fixes 
in order to be really usable.

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[PATCH v4 5/5] i3c: cdns: use parity8 helper instead of open coding it

[Wolfram Sang]

The kernel has now a generic helper for getting parity with easier to
understand semantics. Make use of it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
---
 drivers/i3c/master/i3c-master-cdns.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/i3c/master/i3c-master-cdns.c b/drivers/i3c/master/i3c-master-cdns.c
index 06c0592487d3..fedbe6624a1c 100644
--- a/drivers/i3c/master/i3c-master-cdns.c
+++ b/drivers/i3c/master/i3c-master-cdns.c
@@ -889,8 +889,7 @@ static u32 prepare_rr0_dev_address(u32 addr)
 	ret |= (addr & GENMASK(9, 7)) << 6;
 
 	/* RR0[0] = ~XOR(addr[6:0]) */
-	if (!(hweight8(addr & 0x7f) & 1))
-		ret |= 1;
+	ret |= parity8(addr & 0x7f) ? 0 : BIT(0);
 
 	return ret;
 }
-- 
2.45.2


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Re: [PATCH v4 0/5] i3c: introduce and use generic parity helper

[Wolfram Sang]

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> A build-tested branch is here:

That was locally built. Now, build bot said it is happy, too.


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Re: [PATCH v4 0/5] i3c: introduce and use generic parity helper

[Alexandre Belloni]

On Tue, 07 Jan 2025 10:01:58 +0100, Wolfram Sang wrote:
> Changes since v3:
> 
> * updated commit message of patch 4 to state that a bug gets fixed.
> 
> All acks from bitmap.h and HWMON maintainers are already included.
> 
> Old coverletter follows:
> 
> [...]

Applied, thanks!

[1/5] bitops: add generic parity calculation for u8
      https://git.kernel.org/abelloni/c/c320592f3f2a
[2/5] hwmon: (spd5118) Use generic parity calculation
      https://git.kernel.org/abelloni/c/32a8d362b515
[3/5] i3c: dw: use parity8 helper instead of open coding it
      https://git.kernel.org/abelloni/c/e89cc14e96a9
[4/5] i3c: mipi-i3c-hci: use parity8 helper instead of open coding it
      https://git.kernel.org/abelloni/c/e55905a3f33c
[5/5] i3c: cdns: use parity8 helper instead of open coding it
      https://git.kernel.org/abelloni/c/5e8c732357ce

Best regards,

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