From: Matti Vaittinen <mazziesaccount@gmail.com>

The BD71828 power-supply driver assumes register addresses to be 8-bit.
The new BD72720 will use stacked register maps to hide paging which is
done using secondary I2C slave address. This requires use of 9-bit
register addresses in the power-supply driver (added offset 0x100 to
the 8-bit hardware register addresses).

The cost is slightly used memory consumption as the members in the
struct pwr_regs will be changed from u8 to unsigned int, which means 3
byte increase / member / instance.
This is currently 14 members (expected to possibly be increased when
adding new variants / new functionality which may introduce new
registers, but not expected to grow much) and 2 instances (will be 3
instances when BD72720 gets added).

So, even if the number of registers grew to 50 it'd be 150 bytes /
instance. Assuming we eventually supported 5 variants, it'd be
5 * 150 bytes, which stays very reasonable considering systems we are
dealing with.

As a side note, we can reduce the "wasted space / member / instance" from
3 bytes to 1 byte, by using u16 instead of the unsigned int if needed. I
rather use unsigned int to be initially prepared for devices with 32 bit
registers if there is no need to count bytes.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
---
Revision history:
 v2 => :
 - No changes

 RFCv1 => v2:
 - New patch
---
 drivers/power/supply/bd71828-power.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/drivers/power/supply/bd71828-power.c b/drivers/power/supply/bd71828-power.c
index f667baedeb77..ce73c0f48397 100644
--- a/drivers/power/supply/bd71828-power.c
+++ b/drivers/power/supply/bd71828-power.c
@@ -44,19 +44,19 @@
 #define VBAT_LOW_TH			0x00D4
 
 struct pwr_regs {
-	u8 vbat_avg;
-	u8 ibat;
-	u8 ibat_avg;
-	u8 btemp_vth;
-	u8 chg_state;
-	u8 bat_temp;
-	u8 dcin_stat;
-	u8 dcin_collapse_limit;
-	u8 chg_set1;
-	u8 chg_en;
-	u8 vbat_alm_limit_u;
-	u8 conf;
-	u8 vdcin;
+	unsigned int vbat_avg;
+	unsigned int ibat;
+	unsigned int ibat_avg;
+	unsigned int btemp_vth;
+	unsigned int chg_state;
+	unsigned int bat_temp;
+	unsigned int dcin_stat;
+	unsigned int dcin_collapse_limit;
+	unsigned int chg_set1;
+	unsigned int chg_en;
+	unsigned int vbat_alm_limit_u;
+	unsigned int conf;
+	unsigned int vdcin;
 };
 
 static const struct pwr_regs pwr_regs_bd71828 = {
-- 
2.51.1

On Thu, 13 Nov 2025 10:55:39 +0200
Matti Vaittinen <matti.vaittinen@linux.dev> wrote:

> As a side note, we can reduce the "wasted space / member / instance" from
> 3 bytes to 1 byte, by using u16 instead of the unsigned int if needed. I
> rather use unsigned int to be initially prepared for devices with 32 bit
> registers if there is no need to count bytes.

Well, this is totally internal to the module, so no ABI/API changes, so
there is no advantage of using 32bit now I think. We can switch any time.
But we have 32bit stuff in the regmap cache anyways, so that is not above
the general level of wasting space.

Regards,
Andreas

Thanks Andreas,

On 14/11/2025 13:15, Andreas Kemnade wrote:
> On Thu, 13 Nov 2025 10:55:39 +0200
> Matti Vaittinen <matti.vaittinen@linux.dev> wrote:
> 
>> As a side note, we can reduce the "wasted space / member / instance" from
>> 3 bytes to 1 byte, by using u16 instead of the unsigned int if needed. I
>> rather use unsigned int to be initially prepared for devices with 32 bit
>> registers if there is no need to count bytes.
> 
> Well, this is totally internal to the module, so no ABI/API changes, so
> there is no advantage of using 32bit now I think. We can switch any time.

The only advantage is to avoid the churn if 32bit ICs are to be added.

> But we have 32bit stuff in the regmap cache anyways, so that is not above
> the general level of wasting space.

Exactly. And, I am not sure if sparing ~hundred bytes is worth the 
hassle - even if it is hassle internal to the driver. But yeah, we can 
squeeze a few bytes if it is seen beneficial. That's why I mentioned it 
here :)

Yours,
	-- Matti

---
Matti Vaittinen
Linux kernel developer at ROHM Semiconductors
Oulu Finland

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