Re: [PATCH 28/57] media: Add ovxxxx_16bit_addr_reg_helpers.h

Sakari Ailus
2023-02-10T12:17:22Z

Hi Laurent,

On Fri, Feb 10, 2023 at 02:09:02PM +0200, Laurent Pinchart wrote:
> Hi Hans,
>
> On Fri, Feb 10, 2023 at 12:56:45PM +0100, Hans de Goede wrote:
> > On 2/10/23 12:45, Laurent Pinchart wrote:
> > > On Fri, Feb 10, 2023 at 12:20:36PM +0100, Hans de Goede wrote:
> > >> On 2/9/23 17:11, Laurent Pinchart wrote:
> > >>> On Thu, Feb 09, 2023 at 04:03:22PM +0100, Hans de Goede wrote:
> > >>>> On 2/8/23 10:52, Laurent Pinchart wrote:
> > >>>>> On Mon, Jan 23, 2023 at 01:51:36PM +0100, Hans de Goede wrote:
> > >>>>>> The following drivers under drivers/media/i2c: ov08x40.c, ov13858.c,
> > >>>>>> ov13b10.c, ov2680.c, ov2685.c, ov2740.c, ov4689.c, ov5670.c,
> > >>>>>> ov5675.c, ov5695.c, ov8856.c, ov9282.c and ov9734.c,
> > >>>>>>
> > >>>>>> as well as various "atomisp" sensor drivers in drivers/staging, *all*
> > >>>>>> use register access helpers with the following function prototypes:
> > >>>>>>
> > >>>>>> int ovxxxx_read_reg(struct ovxxxx_dev *sensor, u16 reg,
> > >>>>>> unsigned int len, u32 *val);
> > >>>>>>
> > >>>>>> int ovxxxx_write_reg(struct ovxxxx_dev *sensor, u16 reg,
> > >>>>>> unsigned int len, u32 val);
> > >>>>>>
> > >>>>>> To read/write registers on Omnivision OVxxxx image sensors wich expect
> > >>>>>> a 16 bit register address in big-endian format and which have 1-3 byte
> > >>>>>> wide registers, in big-endian format (for the higher width registers).
> > >>>>>>
> > >>>>>> Add a new ovxxxx_16bit_addr_reg_helpers.h header file with static inline
> > >>>>>> versions of these register access helpers, so that this code duplication
> > >>>>>> can be removed.
> > >>>>>
> > >>>>> Any reason to hand-roll those instead of using regmap ?
> > >>>>
> > >>>> These devices have a mix of 8 + 16 + 24 bit registers which regmap
> > >>>> appears to not handle, a regmap has a single regmap_config struct
> > >>>> with a single "@reg_bits: Number of bits in a register address, mandatory",
> > >>>> so we would still need wrappers around regmap, at which point it
> > >>>> really offers us very little.
> > >>>
> > >>> We could extend regmap too, although that may be too much yak shaving.
> > >>> It would be nice, but I won't push hard for it.
> > >>>
> > >>>> Also I'm moving duplicate code present in many of the
> > >>>> drivers/media/i2c/ov*.c files into a common header to remove
> > >>>> duplicate code. The handrolling was already there before :)
> > >>>>
> > >>>> My goal with the new ovxxxx_16bit_addr_reg_helpers.h file was to
> > >>>> offer something which is as much of a drop-in replacement of the
> > >>>> current handrolled code as possible (usable with just a few
> > >>>> search-n-replaces) as possible.
> > >>>>
> > >>>> Basically my idea here was to factor out code which I noticed was
> > >>>> being repeated over and over again. My goal was not to completely
> > >>>> redo how register accesses are done in these drivers.
> > >>>>
> > >>>> I realize I have not yet converted any other drivers, that is because
> > >>>> I don't really have a way to test most of the other drivers. OTOH
> > >>>> with the current helpers most conversions should be fairly simply
> > >>>> and remove a nice amount of code. So maybe I should just only compile
> > >>>> test the conversions ?
> > >>>
> > >>> Before you spend time converting drivers, I'd like to complete the
> > >>> discussion regarding the design of those helpers. I'd rather avoid
> > >>> mass-patching drivers now and doing it again in the next kernel release.
> > >>
> > >> I completely agree.
> > >>
> > >>> Sakari mentioned CCI (part of the CSI-2 specification). I think that
> > >>> would be a good name to replace ov* here, as none of this is specific to
> > >>> OmniVision.
> > >>
> > >> I did not realize this was CCI I agree renaming the helpers makes sense.
> > >>
> > >> I see there still is a lot of discussion going on.
> > >
> > > I haven't seen any disagreement regarding the cci prefix, so let's go
> > > for that. I'd propose cci_read() and cci_write().
> > >
> > > Sakari, you and I would prefer layering this on top of regmap, while
> > > Andy proposed extending the regmap API. Let's see if we reach an
> > > anonymous agreement on this.
> > >
> > > Regarding the width-specific versions of the helpers, I really think
> > > encoding the size in the register macros is the best option. It makes
> > > life easier for driver authors (only one function to call, no need to
> > > think about the register width to pick the appropriate function in each
> > > call) and reviewers (same reason), without any drawback in my opinion.
> > >
> > > Another feature I'd like in these helpers is improved error handling. In
> > > quite a few sensor drivers I've written, I've implemented the write
> > > function as
> > >
> > > int foo_write(struct foo *foo, u32 reg, u32 val, int *err)
> > > {
> > > ...
> > > int ret;
> > >
> > > if (err && *err)
> > > return *err;
> > >
> > > ret = real_write(...);
> > > if (ret < 0) {
> > > dev_err(...);
> > > if (err)
> > > *err = ret;
> > > }
> > >
> > > return ret;
> > > }
> > >
> > > This allows callers to write
> > >
> > > int ret = 0;
> > >
> > > foo_write(foo, REG_A, 0, &ret);
> > > foo_write(foo, REG_B, 1, &ret);
> > > foo_write(foo, REG_C, 2, &ret);
> > > foo_write(foo, REG_D, 3, &ret);
> > >
> > > return ret;
> > >
> > > which massively simplifies error handling. I'd like the CCI write helper
> > > to implement such a pattern.
> >
> > Interesting, I see that the passing of the err return pointer is optional,
> > so we can still just do a search replace in existing code setting that
> > to just NULL.
>
> And if someone dislikes having to pass NULL for the last argument, we
> could use some macro magic to accept both the 3 arguments and 4
> arguments variants.
>
> int __cci_write3(struct cci *cci, u32 reg, u32 val);
> int __cci_write4(struct cci *cci, u32 reg, u32 val, int *err);
>
> #define __cci_write(_1, _2, _3, _4, NAME, ...) NAME
> #define cci_write(...) __cci_write(__VA_ARGS__, __cci_write4, __cci_write3)(__VA_ARGS__)

This would be nice, yes.

Who will now write the patches for this? :-)

--
Sakari Ailus