Re: [PATCH v4 6/8] media: imx274: add helper function to fill a reg_8 table chunk

From: Sakari Ailus
Date: Tue Jun 26 2018 - 08:21:09 EST

Hi Luca,

On Mon, Jun 11, 2018 at 01:35:37PM +0200, Luca Ceresoli wrote:
> Tables of struct reg_8 are used to simplify multi-byte register
> assignment. However filling these snippets with values computed at
> runtime is currently implemented by very similar functions doing the
> needed shift & mask manipulation.
>
> Replace all those functions with a unique helper function to fill
> reg_8 tables in a simple and clean way.

What's the purpose of writing these registers as multiple I²C writes, when
this can be done as a single write (i.e. the address followed by two or
three octets of data)?

>
> Signed-off-by: Luca Ceresoli <luca@xxxxxxxxxxxxxxxx>
> Cc: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
>
> ---
> Changed v3 -> v4: nothing
>
> Changed v2 -> v3:
> - minor reformatting in prepare_reg() documentation
>
> Changed v1 -> v2:
> - add "media: " prefix to commit message
> ---
> drivers/media/i2c/imx274.c | 90 ++++++++++++++++++++++++++++------------------
> 1 file changed, 55 insertions(+), 35 deletions(-)
>
> diff --git a/drivers/media/i2c/imx274.c b/drivers/media/i2c/imx274.c
> index 48343c2ade83..e5ba19b97083 100644
> --- a/drivers/media/i2c/imx274.c
> +++ b/drivers/media/i2c/imx274.c
> @@ -597,6 +597,58 @@ static inline struct stimx274 *to_imx274(struct v4l2_subdev *sd)
> }
>
> /*
> + * Fill consecutive reg_8 items in order to set a multibyte register.
> + *
> + * The sensor has many 2-bytes registers and a 3-byte register. This
> + * simplifies code to set them by filling consecutive entries of a
> + * struct reg_8 table with the data to set a register.
> + *
> + * The number of table entries that is filled is the minimum needed
> + * for the given number of bits (i.e. nbits / 8, rounded up). If nbits
> + * is not a multiple of 8, extra bits in the most significant byte are
> + * zeroed.
> + *
> + * Example:
> + * Calling prepare_reg(&regs[10], 0x3000, 0xcafe, 12) will set:
> + * regs[10] = { .addr = 0x3000, .val = 0xfe }
> + * regs[11] = { .addr = 0x3001, .val = 0x0a }
> + *
> + * @table_base: Pointer to the first reg_8 struct to be filled. The
> + * following entries will also be set, make sure they are
> + * properly allocated!
> + * @addr_lsb: Address of the LSB register. Other registers must be
> + * consecutive, least-to-most significant.
> + * @value: Value to be written to the register.
> + * @nbits: Number of bits to write (range: [1..24])
> + */
> +static void prepare_reg(struct reg_8 *table_base,
> + u16 addr_lsb,
> + u32 value,
> + size_t nbits)
> +{
> + struct reg_8 *cmd = table_base;
> + u16 addr = addr_lsb;
> + size_t bits; /* how many bits at this round */
> +
> + WARN_ON(nbits > 24);
> +
> + if (nbits > 24)
> + nbits = 24;
> +
> + while (nbits > 0) {
> + bits = min_t(size_t, 8, nbits);
> +
> + cmd->addr = addr;
> + cmd->val = value & ((1 << bits) - 1);
> +
> + nbits -= bits;
> + value >>= 8;
> + addr++;
> + cmd++;
> + }
> +}
> +
> +/*
> * Writing a register table
> *
> * @priv: Pointer to device
> @@ -1163,15 +1215,6 @@ static int imx274_set_digital_gain(struct stimx274 *priv, u32 dgain)
> reg_val & IMX274_MASK_LSB_4_BITS);
> }
>
> -static inline void imx274_calculate_gain_regs(struct reg_8 regs[2], u16 gain)
> -{
> - regs->addr = IMX274_ANALOG_GAIN_ADDR_MSB;
> - regs->val = (gain >> IMX274_SHIFT_8_BITS) & IMX274_MASK_LSB_3_BITS;
> -
> - (regs + 1)->addr = IMX274_ANALOG_GAIN_ADDR_LSB;
> - (regs + 1)->val = (gain) & IMX274_MASK_LSB_8_BITS;
> -}
> -
> /*
> * imx274_set_gain - Function called when setting gain
> * @priv: Pointer to device structure
> @@ -1229,7 +1272,7 @@ static int imx274_set_gain(struct stimx274 *priv, struct v4l2_ctrl *ctrl)
> if (gain_reg > IMX274_GAIN_REG_MAX)
> gain_reg = IMX274_GAIN_REG_MAX;
>
> - imx274_calculate_gain_regs(reg_list, (u16)gain_reg);
> + prepare_reg(reg_list, IMX274_ANALOG_GAIN_ADDR_LSB, gain_reg, 11);
>
> for (i = 0; i < ARRAY_SIZE(reg_list); i++) {
> err = imx274_write_reg(priv, reg_list[i].addr,
> @@ -1258,16 +1301,6 @@ static int imx274_set_gain(struct stimx274 *priv, struct v4l2_ctrl *ctrl)
> return err;
> }
>
> -static inline void imx274_calculate_coarse_time_regs(struct reg_8 regs[2],
> - u32 coarse_time)
> -{
> - regs->addr = IMX274_SHR_REG_MSB;
> - regs->val = (coarse_time >> IMX274_SHIFT_8_BITS)
> - & IMX274_MASK_LSB_8_BITS;
> - (regs + 1)->addr = IMX274_SHR_REG_LSB;
> - (regs + 1)->val = (coarse_time) & IMX274_MASK_LSB_8_BITS;
> -}
> -
> /*
> * imx274_set_coarse_time - Function called when setting SHR value
> * @priv: Pointer to device structure
> @@ -1292,7 +1325,7 @@ static int imx274_set_coarse_time(struct stimx274 *priv, u32 *val)
> goto fail;
>
> /* prepare SHR registers */
> - imx274_calculate_coarse_time_regs(reg_list, coarse_time);
> + prepare_reg(reg_list, IMX274_SHR_REG_LSB, coarse_time, 16);
>
> /* write to SHR registers */
> for (i = 0; i < ARRAY_SIZE(reg_list); i++) {
> @@ -1429,19 +1462,6 @@ static int imx274_set_test_pattern(struct stimx274 *priv, int val)
> return err;
> }
>
> -static inline void imx274_calculate_frame_length_regs(struct reg_8 regs[3],
> - u32 frame_length)
> -{
> - regs->addr = IMX274_VMAX_REG_1;
> - regs->val = (frame_length >> IMX274_SHIFT_16_BITS)
> - & IMX274_MASK_LSB_4_BITS;
> - (regs + 1)->addr = IMX274_VMAX_REG_2;
> - (regs + 1)->val = (frame_length >> IMX274_SHIFT_8_BITS)
> - & IMX274_MASK_LSB_8_BITS;
> - (regs + 2)->addr = IMX274_VMAX_REG_3;
> - (regs + 2)->val = (frame_length) & IMX274_MASK_LSB_8_BITS;
> -}
> -
> /*
> * imx274_set_frame_length - Function called when setting frame length
> * @priv: Pointer to device structure
> @@ -1463,7 +1483,7 @@ static int imx274_set_frame_length(struct stimx274 *priv, u32 val)
>
> frame_length = (u32)val;
>
> - imx274_calculate_frame_length_regs(reg_list, frame_length);
> + prepare_reg(reg_list, IMX274_VMAX_REG_3, frame_length, 20);
> for (i = 0; i < ARRAY_SIZE(reg_list); i++) {
> err = imx274_write_reg(priv, reg_list[i].addr,
> reg_list[i].val);
> --
> 2.7.4
>

--
Sakari Ailus
e-mail: sakari.ailus@xxxxxx