Re: [PATCH v2 1/2] media: v4l2-cci: Add support for little-endian encoded registers

[Sakari Ailus <sakari.ailus@linux.intel.com>]

Hi Alexander,

On Thu, Nov 02, 2023 at 08:51:12AM +0100, Alexander Stein wrote:
> Hi,
> 
> thanks for the feedback.
> 
> Am Donnerstag, 2. November 2023, 07:30:44 CET schrieb Sakari Ailus:
> > Hi Laurent,
> > 
> > On Thu, Nov 02, 2023 at 03:22:17AM +0200, Laurent Pinchart wrote:
> > > Hi Alexander,
> > > 
> > > Thank you for the patch.
> > > 
> > > On Wed, Nov 01, 2023 at 01:23:53PM +0100, Alexander Stein wrote:
> > > > Some sensors, e.g. Sony, are using little-endian registers. Add support
> > > > for
> > > 
> > > I would write Sony IMX290 here, as there are Sony sensors that use big
> > > endian.
> > 
> > Almost all of them. There are a few exceptions indeed. This seems to be a
> > bug.
> 
> Let's name IMX290 here as an actual example. No need to worry about other 
> models here.
> 
> > > > those by encoding the endianess into Bit 20 of the register address.
> > > > 
> > > > Fixes: af73323b97702 ("media: imx290: Convert to new CCI register access
> > > > helpers") Cc: stable@vger.kernel.org
> > > > Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
> > > > ---
> > > > 
> > > >  drivers/media/v4l2-core/v4l2-cci.c | 44 ++++++++++++++++++++++++------
> > > >  include/media/v4l2-cci.h           |  5 ++++
> > > >  2 files changed, 41 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/drivers/media/v4l2-core/v4l2-cci.c
> > > > b/drivers/media/v4l2-core/v4l2-cci.c index bc2dbec019b04..673637b67bf67
> > > > 100644
> > > > --- a/drivers/media/v4l2-core/v4l2-cci.c
> > > > +++ b/drivers/media/v4l2-core/v4l2-cci.c
> > > > @@ -18,6 +18,7 @@
> > > > 
> > > >  int cci_read(struct regmap *map, u32 reg, u64 *val, int *err)
> > > >  {
> > > > 
> > > > +	bool little_endian;
> > > > 
> > > >  	unsigned int len;
> > > >  	u8 buf[8];
> > > >  	int ret;
> > > > 
> > > > @@ -25,6 +26,7 @@ int cci_read(struct regmap *map, u32 reg, u64 *val,
> > > > int *err)> > 
> > > >  	if (err && *err)
> > > >  	
> > > >  		return *err;
> > > > 
> > > > +	little_endian = reg & CCI_REG_LE;
> > > 
> > > You could initialize the variable when declaring it. Same below.
> > 
> > I was thinking of the same, but then it'd be logical to move initialisation
> > of all related variables there. reg is modified here though. I'd keep
> > setting little_endian here. If someone wants to move it, that could be done
> > in a separate patch.
> > 
> > > >  	len = FIELD_GET(CCI_REG_WIDTH_MASK, reg);
> > > >  	reg = FIELD_GET(CCI_REG_ADDR_MASK, reg);
> > > > 
> > > > @@ -40,16 +42,28 @@ int cci_read(struct regmap *map, u32 reg, u64 *val,
> > > > int *err)> > 
> > > >  		*val = buf[0];
> > > >  		break;
> > > >  	
> > > >  	case 2:
> > > > -		*val = get_unaligned_be16(buf);
> > > > +		if (little_endian)
> > > > +			*val = get_unaligned_le16(buf);
> > > > +		else
> > > > +			*val = get_unaligned_be16(buf);
> > > 
> > > Unrelated to this patch, isn't buf aligned to a 4 bytes boundary ?
> > 
> > Very probably, as it's right after len that's an unsigned int. Adding
> > __aligned(8) would ensure we don't need any of the unaligned variants, and
> > most likely would keep the stack layout as-is.
> 
> You mean something like this?
> 
> u8 __aligned(8) buf[8];
> [...]
> if (little_endian)
> 	*val = le64_to_cpup(buf);
> else
> 	*val = be64_to_cpup(buf);
> 
> But what about 24 Bits? There is no le24_to_cpup. I would rather use the same 
> API for all cases.

The aligned APIs are much better choice when you can use them. The 24 bit
case can remain special IMO.

> 
> > Or... how about putting it in an union with a u64? That would mean it's
> > accessible as u64 alignment-wise while the alignment itself is up to the
> > ABI. A comment would be good to have probably.
> 
> An additional union seems a bit too much here. Unless something suitable 
> already exists for general usage.

I think it's nicer than using __aligned() as you get ABI alignment that
way, not something you force manually --- that's a bit crude.

I wonder that others think.

-- 
Regards,

Sakari Ailus