Re: [PATCH RFC 0/6] phy/media: Add generic MIPI C-PHY support to CSI-2 stack

[Bryan O'Donoghue]

On 17/10/2026 18:13, Nihal Kumar Gupta wrote:
> George posted a C-PHY series [1] which we are currently reviewing.
> As discussed with Bryan, this series shares our independent
> implementation so the approaches can be compared.
>
> Two aspects covered here are not in [1]:
>
> - Settle count calculation derived from sensor-reported T_PREPARE and
>    T_PREAMBLE timing via get_mbus_config(). [1] left this as an open
>    question.
>
> - Data rate based register sequence selection using effective bit
>    rate (link_freq * 32 / 7). [1] noted downstream does this but
>    deferred it; this series implements it.
>
> MIPI CSI-2 operates over two physical layer variants. D-PHY uses a
> dedicated differential clock lane with up to four data lanes. C-PHY
> encodes the clock within the data stream across up to three wire
> trios, eliminating the dedicated clock lane.
>
> Patch 1 extends the generic PHY framework with PHY_MODE_MIPI_CPHY
> and struct phy_configure_opts_mipi_cphy.
>
> Patch 2 adds t_prepare_ns and t_preamble_ns fields to the CSI-2
> media bus configuration and extends CAMSS to forward them to the PHY
> provider for settle count calculation. Sensor drivers must implement
> get_mbus_config() and return non-zero values in nanoseconds; stream-on
> returns -EINVAL otherwise. The strictness is deliberate: C-PHY settle
> timing is symbol-rate-range specific and no safe value can be derived
> from link frequency alone. A conservative fallback could be added if
> reviewers prefer.
>
> Patches 3-6 extend the Qualcomm CSIPHY driver: refactor D-PHY
> register storage to accommodate C-PHY, accept C-PHY mode with
> multiple data rate table selection, compute settle count, and add
> lane power mask support for C-PHY trios.
>
> C-PHY encodes 16 bits per 7 symbols, so the effective bit rate is:
>
>    bit_rate = link_freq * 2 * 16 / 7 = link_freq * 32 / 7
>
> where link_freq is symbol_rate / 2 as reported by the sensor via
> get_mbus_config(). The rate-table entry whose bandwidth first meets
> or exceeds bit_rate is selected.
>
> The settle count defines how many timer clock cycles the CSIPHY
> receiver waits before scanning for C-PHY sync. It is computed from
> the sensor's T_PREPARE and T_PREAMBLE timing. The /3 factor reflects
> that C-PHY distributes the preamble sequence across three trios:
>
>    settle_cnt = (t_prepare_ns + t_preamble_ns / 3) * timer_clk_rate / 1e9
>
> D-PHY settle count is unchanged from before this series.
>
> [1] https://lore.kernel.org/all/20261016-x1e-cphy-rfc-v3-0-40a346a72153@xxxxxxxxx/
>
> Signed-off-by: Nihal Kumar Gupta <nihal.gupta@xxxxxxxxxxxxxxxx>
> ---
> Nihal Kumar Gupta (6):
>        phy: core: Add MIPI C-PHY configurations
>        media: qcom: camss: Add C-PHY timing fields and get config from sensor
>        phy: qcom: mipi-csi2: Split D-PHY register settings
>        phy: qcom: mipi-csi2: Accept C-PHY mode and add register sequence
>        phy: qcom: mipi-csi2: Add C-PHY settle calculation
>        phy: qcom: mipi-csi2: Add lane mask calculation for C-PHY
>
>   drivers/media/platform/qcom/camss/camss-csiphy.c   |  90 +++++++++---
>   drivers/media/platform/qcom/camss/camss-csiphy.h   |   2 +
>   drivers/media/platform/qcom/camss/camss.c          |   8 +-
>   drivers/phy/qualcomm/phy-qcom-mipi-csi2-3ph-dphy.c | 159 +++++++++++++++++----

So on thing I notice about this series and like is that there's a small 
change to the existing 3ph-dphy.c

- Since it supports 3ph-cphy the file should be renamed
  3ph-cphy-dphy.c (or something)
- I like the progressive nature of he changes in this series
  as structurally the right way to add stuff in. Small digestable
  commits
- I think the missing piece here is data_lanes[] and lane_polarities[]
  per the other series from Qcom
- Another thing is the other series from Anusha has
  a more advanced mipi_ops data-structure for CPHY.

Basically I like the approach taken in this series for a smaller change 
to the phy-qcom-mipi-csi2-3ph-dphy.c file.

I'd like the best of both series - I think there is alot to resolve on 
lanes, data-lanes and lane-polarities in the other series and looking at 
this series, it seems to me we don't need a separate 
phy-qcom-mipi-csi2-3ph-cphy.c, I think Konrad observed in George's 
series that really the file deals with 3-phase PHYs and that seems true 
enough.

Please work out between yourself and Anusha what's next.

If possible I would prefer to contain 3ph CPHY and DPHY to one file as 
is done here - at the same time the definition of CPHY ops and lane 
mappings is closer to the spec in the other series.

I think either you should take Anusha's structs and lane definitions or 
Anusha should take your approach to minimal change to the 
phy-qcom-mipi-csi2-3ph-dphy.c renaming it.

One of you needs to pick up the patches from the other one.

George can land his stuff on top then.
>   drivers/phy/qualcomm/phy-qcom-mipi-csi2-core.c     |  51 +++++--
>   drivers/phy/qualcomm/phy-qcom-mipi-csi2.h          |  25 +++-
>   include/linux/phy/phy.h                            |  47 ++++++
>   include/media/v4l2-mediabus.h                      |   2 +
>   8 files changed, 315 insertions(+), 69 deletions(-)
> ---
> base-commit: 2efb7a11f1aa4d2dc285fe2680cca22cb9f72f64
> change-id: 20261017-cphy_support-2ce4d656e631
>
> Best regards,
> --
> Nihal Kumar Gupta <nihal.gupta@xxxxxxxxxxxxxxxx>